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The '''Fourier transform''' ('''FT''') decomposes a [[function (mathematics)|function]] of time (a ''signal'') into its constituent frequencies.  This is similar to the way a musical [[Chord (music)|chord]] can be expressed in terms of the volumes and frequencies of its constituent notes. The term ''Fourier transform'' refers to both the [[frequency domain]] representation and the mathematical operation that associates the frequency domain representation to a function of [[Time in physics|time]]. The Fourier transform of a function of time is itself a [[complex number|complex]]-valued function of frequency, whose magnitude ([[absolute value|modulus]]) represents the amount of that frequency present in the original function, and whose [[argument (complex analysis)|argument]] is the [[phase offset]] of the basic sinusoid in that frequency. The Fourier transform is not limited to functions of time, but the [[Domain of a function|domain of the original function]] is commonly referred to as the ''[[time domain]]''.  There is also an ''inverse Fourier transform'' that mathematically synthesizes the original function from its frequency domain representation.
<!--Sorry... I need more explanation than the following statement.  So you can be sure that a majority of readers also do.
This can then be displayed as a graphic, with the time domain replaced by a ''spatial domain''. In this representation, the intensity of a signal is identified by its spatial position rather than a specific point in time.-->

{{Gallery|width=150 | height=150 |lines=2 |align=right
|File:Sine voltage.svg
 |A [[Sine wave|sinusoidal]] curve, with peak amplitude (1), peak-to-peak (2), [[Root mean square|RMS]] (3), and wave period (4).
|File:Phase shift.svg
 |Illustration of phase shift {{mvar|θ}}.
}}

{{ Annotated image | caption=In the first row of the figure is the graph of the [[unit pulse]] function {{math|''f''&thinsp;(''t'')}} and its Fourier transform {{math|''f̂''&thinsp;(''ω'')}}, a function of frequency {{mvar|ω}}. Translation (that is, delay) in the time domain is interpreted as complex phase shifts in the frequency domain. In the second row is shown {{math|''g''(''t'')}}, a delayed unit pulse, beside the real and imaginary parts of the Fourier transform. The Fourier transform decomposes a function into eigenfunctions for the group of translations.
| image=Fourier_unit_pulse.svg
| image-width = 300
| annotations =
{{Annotation|20|40|<math>\scriptstyle f(t)</math>}}
{{Annotation|170|40|<math>\scriptstyle \hat{f}(\omega)</math>}}
{{Annotation|20|140|<math>\scriptstyle g(t)</math>}}
{{Annotation|170|140|<math>\scriptstyle \hat{g}(\omega)</math>}}
{{Annotation|130|80|<math>\scriptstyle t</math>}}
{{Annotation|280|85|<math>\scriptstyle \omega</math>}}
{{Annotation|130|192|<math>\scriptstyle t</math>}}
{{Annotation|280|180|<math>\scriptstyle \omega</math>}}
}}
{{Fourier transforms}}

Linear operations performed in one domain (time or frequency) have corresponding operations in the other domain, which are sometimes easier to perform. The operation of [[derivative|differentiation]] in the time domain corresponds to multiplication by the frequency,<ref group="remark">Up to an imaginary constant factor whose magnitude depends on what Fourier transform convention is used.</ref> so some [[differential equation]]s are easier to analyze in the frequency domain. Also, [[convolution]] in the time domain corresponds to ordinary multiplication in the frequency domain (see [[Convolution theorem]]). After performing the desired operations, transformation of the result can be made back to the time domain. [[Harmonic analysis]] is the systematic study of the relationship between the frequency and time domains, including the kinds of functions or operations that are "simpler" in one or the other, and has deep connections to many areas of modern mathematics.

Functions that are localized in the time domain have Fourier transforms that are spread out across the frequency domain and vice versa, a phenomenon known as the [[#Uncertainty principle|uncertainty principle]]. The [[critical point (mathematics)|critical]] case for this principle is the [[Gaussian function]], of substantial importance in [[probability theory]] and [[statistics]] as well as in the study of physical phenomena exhibiting [[normal distribution]] (e.g., [[diffusion]]). The Fourier transform of a Gaussian function is another Gaussian function. [[Joseph Fourier]] introduced the transform in his study of [[heat transfer]], where Gaussian functions appear as solutions of the [[heat equation]].

The Fourier transform can be formally defined as an [[improper integral|improper]] [[Riemann integral]], making it an [[integral transform]], although this definition is not suitable for many applications requiring a more sophisticated integration theory.<ref group="remark">Depending on the application a [[Lebesgue integral]], [[distribution (mathematics)|distributional]], or other approach may be most appropriate.</ref> For example, many relatively simple applications use the [[Dirac delta function]], which can be treated formally as if it were a function, but the justification requires a mathematically more sophisticated viewpoint.<ref group="remark">{{harvtxt|Vretblad|2000}} provides solid justification for these formal procedures without going too deeply into [[functional analysis]] or the [[distribution (mathematics)|theory of distributions]].</ref> The Fourier transform can also be generalized to functions of several variables on Euclidean space, sending a function of {{nowrap|3-dimensional}} 'position space' to a function of {{nowrap|3-dimensional}} momentum (or a function of space and time to a function of [[4-momentum]]). This idea makes the spatial Fourier transform very natural in the study of waves, as well as in [[quantum mechanics]], where it is important to be able to represent wave solutions as functions of either position or momentum and sometimes both. In general, functions to which Fourier methods are applicable are complex-valued, and possibly [[vector-valued function|vector-valued]].<ref group="remark">In [[relativistic quantum mechanics]] one encounters vector-valued Fourier transforms of multi-component wave functions. In [[quantum field theory]], operator-valued Fourier transforms of operator-valued functions of spacetime are in frequent use, see for example {{harvtxt|Greiner|Reinhardt|1996}}.</ref> Still further generalization is possible to functions on [[group (mathematics)|groups]], which, besides the original Fourier transform on {{math|'''ℝ'''}} or {{math|'''ℝ'''<sup>''n''</sup>}} (viewed as groups under addition), notably includes the [[discrete-time Fourier transform]] (DTFT, group = [[integers|{{math|'''ℤ'''}}]]), the [[discrete Fourier transform]] (DFT, group = [[cyclic group|{{math|'''ℤ''' mod ''N''}}]]) and the [[Fourier series]] or circular Fourier transform (group = [[circle group|{{math|''S''<sup>1</sup>}}]], the unit circle ≈ closed finite interval with endpoints identified). The latter is routinely employed to handle [[periodic function]]s. The [[fast Fourier transform]] (FFT) is an algorithm for computing the DFT.

==Definition==
The Fourier transform of a function {{mvar|f}} is traditionally denoted <math> \hat{f}</math>, by adding a [[circumflex]] to the symbol of the function. There are several [[#Other conventions|common conventions]] for defining the Fourier transform of an [[Lebesgue integration|integrable]] function <math>f: \mathbb{R} \to \mathbb{C}</math>.<ref>{{harvnb|Kaiser|1994|p=29}}.</ref><ref>{{harvnb|Rahman|2011|p=11}}.</ref> One of them is

{{Equation box 1
|indent =
|title=
|equation = {{NumBlk||<math>\hat{f}(\xi) = \int_{-\infty}^{\infty} f(x)\ e^{-2\pi i x \xi}\,dx,</math>|{{EquationRef|Eq.1}}}}
|cellpadding= 6
|border
|border colour = #0073CF
|background colour=#F5FFFA}}

for any [[real number]] {{mvar|[[Xi (letter)|ξ]]}}.

A reason for the negative sign in the exponent is that it is common in [[electrical engineering]] to [[phasor|represent]]
by <math>f(x) = e^{2 \pi i \xi_0 x}</math>  a signal with zero initial phase and frequency <math>\xi_0.</math><ref>{{cite web | title = Sign Conventions in Electromagnetic (EM) Waves | url = https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-007-electromagnetic-energy-from-motors-to-lasers-spring-2011/readings/MIT6_007S11_sign.pdf}}</ref><ref group="remark">The function <math>f(x) = \cos(2 \pi \xi_0 x) \equiv \cos(-2 \pi \xi_0 x)</math> is also a signal with frequency <math>\xi_0</math>, but the integral obviously produces identical responses at both <math>\xi_0</math> and <math>-\xi_0</math>, which is also consistent with [[Euler's formula]]:  <math>\cos(2 \pi \xi_0 x) \equiv \tfrac{1}{2} e^{i 2 \pi \xi_0 x} + \tfrac{1}{2} e^{-i 2 \pi \xi_0 x}.</math></ref>  The negative sign convention causes the product <math>e^{2 \pi i \xi_0 x} e^{-2\pi i \xi x}</math> to be 1 (frequency zero) when <math>\xi = \xi_0,</math> causing the integral to diverge. The result is a [[Dirac delta function]] at <math>\xi = \xi_0</math>, which is the only frequency component of the [[sinusoidal]] signal <math>e^{2 \pi i \xi_0 x}.</math>

When the independent variable {{mvar|x}} represents ''time'', the transform variable {{mvar|ξ}} represents [[frequency]] (e.g. if time is measured in seconds, then frequency is in [[hertz]]). Under suitable conditions, {{mvar|f}} is determined by <math>\hat{f}</math> via the inverse transform:

{{Equation box 1
|indent =
|title=
|equation = {{NumBlk||<math>f(x) = \int_{-\infty}^{\infty} \hat f(\xi)\ e^{2 \pi i x \xi}\,d\xi,</math>|{{EquationRef|Eq.2}}}}
|cellpadding= 6
|border
|border colour = #0073CF
|background colour=#F5FFFA}}

for any real number&nbsp;{{mvar|x}}.

The statement that {{mvar|f}} can be reconstructed from <math>\hat{f}</math> is known as the [[Fourier inversion theorem]], and was first introduced in [[Joseph Fourier|Fourier's]] ''Analytical Theory of Heat'',<ref>{{harvnb|Fourier|1822|p=525}}.</ref><ref>{{harvnb|Fourier|1878|p=408}}.</ref> although what would be considered a proof by modern standards was not given until much later.<ref>{{harv|Jordan|1883}} proves on pp.&nbsp;216–226 the [[Fourier inversion theorem#Fourier integral theorem|Fourier integral theorem]] before studying Fourier series.</ref><ref>{{harvnb|Titchmarsh|1986|p=1}}.</ref> The functions {{mvar|f}} and <math>\hat{f}</math> often are referred to as a ''Fourier integral pair'' or ''Fourier transform pair''.<ref>{{harvnb|Rahman|2011|p=10}}.</ref>

For other common conventions and notations, including using the [[angular frequency]] {{mvar|[[Omega|ω]]}} instead of the [[frequency]] {{mvar|[[Xi (letter)|ξ]]}}, see [[Fourier transform#Other conventions|Other conventions]] and [[Fourier transform#Other notations|Other notations]] below. The [[#Fourier transform on Euclidean space|Fourier transform on Euclidean space]] is treated separately, in which the variable {{mvar|x}} often represents position and {{mvar|ξ}} momentum. The conventions chosen in this article are those of [[harmonic analysis]], and are characterized as the unique conventions such that the Fourier transform is both [[Unitary operator|unitary]] on {{math|''L''<sup>2</sup>}} and an algebra homomorphism from {{math|''L''<sup>1</sup>}} to {{math|''L''<sup>∞</sup>}}, without renormalizing the Lebesgue measure.<ref>{{harvnb|Folland|1989}}.</ref>

Many other characterizations of the Fourier transform exist. For example, one uses the [[Stone–von Neumann theorem]]: the Fourier transform is the unique unitary [[intertwiner]] for the symplectic and Euclidean Schrödinger representations of the [[Heisenberg group]].

==History==
{{main|Fourier analysis#History|Fourier series#History}}
In 1822, [[Joseph Fourier]] showed that some functions could be written as an infinite sum of harmonics.<ref>{{harvnb|Fourier|1822}}.</ref>

==Introduction==
{{See also|Fourier analysis}}
[[File:Fourier transform time and frequency domains (small).gif|frame|right|In the first frames of the animation, a function {{mvar|f}} is resolved into Fourier series: a linear combination of sines and cosines (in blue). The component frequencies of these sines and cosines spread across the frequency spectrum, are represented as peaks in the frequency domain (actually [[Dirac delta function]]s, shown in the last frames of the animation). The frequency domain representation of the function, {{math|''f̂''}}, is the collection of these peaks at the frequencies that appear in this resolution of the function.]]
One motivation for the Fourier transform comes from the study of [[Fourier series]]. In the study of Fourier series, complicated but periodic functions are written as the sum of simple waves mathematically represented by [[sine]]s and [[cosine]]s. The Fourier transform is an extension of the Fourier series that results when the period of the represented function is lengthened and allowed to approach infinity.<ref>{{harvnb|Taneja|2008|p=192}}.</ref>

Due to the properties of sine and cosine, it is possible to recover the amplitude of each wave in a Fourier series using an integral. In many cases it is desirable to use [[Euler's formula]], which states that {{math|''e''<sup>2π''iθ''</sup> {{=}} cos(2π''θ'') + ''i'' sin(2π''θ'')}}, to write Fourier series in terms of the basic waves {{math|''e''<sup>2π''iθ''</sup>}}. This has the advantage of simplifying many of the formulas involved, and provides a formulation for Fourier series that more closely resembles the definition followed in this article. Re-writing sines and cosines as [[complex exponentials]] makes it necessary for the Fourier coefficients to be complex valued. The usual interpretation of this complex number is that it gives both the [[amplitude]] (or size) of the wave present in the function and the [[phase (waves)|phase]] (or the initial angle) of the wave. These complex exponentials sometimes contain negative "frequencies". If {{mvar|θ}} is measured in seconds, then the waves {{math|''e''<sup>2π''iθ''</sup>}} and {{math|''e''<sup>−2π''iθ''</sup>}} both complete one cycle per second, but they represent different frequencies in the Fourier transform. Hence, frequency no longer measures the number of cycles per unit time, but is still closely related.

There is a close connection between the definition of Fourier series and the Fourier transform for functions {{mvar|f}} that are zero outside an interval. For such a function, we can calculate its Fourier series on any interval that includes the points where {{mvar|f}} is not identically zero. The Fourier transform is also defined for such a function. As we increase the length of the interval in which we calculate the Fourier series, then the Fourier series coefficients begin to resemble the Fourier transform and the sum of the Fourier series of {{mvar|f}} begins to resemble the inverse Fourier transform. More precisely, suppose {{mvar|T}} is large enough that the interval {{math|[−{{sfrac|''T''|2}}, {{sfrac|''T''|2}}]}} contains the interval in which {{mvar|f}} is not identically zero. Then, the {{mvar|n}}th series coefficient {{mvar|c<sub>n</sub>}} is given by:

:<math>c_n = \frac{1}{T} \int_{-\frac{T}{2}}^\frac{T}{2} f(x)\, e^{-2\pi i\left(\frac{n}{T}\right) x} \, dx.</math>

Comparing this to the definition of the Fourier transform, it follows that:

:<math>c_n = \frac{1}{T}\hat f\left(\frac{n}{T}\right)</math>

since {{math|''f''&thinsp;(''x'')}} is zero outside {{math|[−{{sfrac|''T''|2}}, {{sfrac|''T''|2}}]}}. Thus, the Fourier coefficients are equal to the values of the Fourier transform sampled on a grid of width {{math|{{sfrac|1|''T''}}}}, multiplied by the grid width {{math|{{sfrac|1|''T''}}}}.

Under appropriate conditions, the Fourier series of {{mvar|f}} will equal the function {{mvar|f}}. In other words, {{mvar|f}} can be written:

:<math>f(x)=\sum_{n=-\infty}^\infty c_n\, e^{2\pi i\left(\frac{n}{T}\right) x} =\sum_{n=-\infty}^\infty \hat{f}(\xi_n)\ e^{2\pi i\xi_n x}\Delta\xi,</math>

where the last sum is simply the first sum rewritten using the definitions {{math|''ξ<sub>n</sub>'' {{=}} ''{{sfrac|n|T}}''}}, and {{math|Δ''ξ'' {{=}} {{sfrac|''n'' + 1|''T''}} − {{sfrac|''n''|''T''}} {{=}} {{sfrac|1|''T''}}}}.

This second sum is a [[Riemann sum]]. By letting {{math|''T'' → ∞}} it will converge to the integral for the inverse Fourier transform as expressed above. Under suitable conditions, this argument may be made precise.<ref name="Stein-Shakarchi-2003">{{harvnb|Stein|Shakarchi|2003}}.</ref>

In the study of Fourier series the numbers {{mvar|c<sub>n</sub>}} could be thought of as the "amount" of the wave present in the Fourier series of {{mvar|f}}. Similarly, as seen above, the Fourier transform can be thought of as a function that measures how much of each individual frequency is present in our function {{mvar|f}}, and we can recombine these waves by using an integral (or "continuous sum") to reproduce the original function.

==Example==
The following figures provide a visual illustration how the Fourier transform measures whether a frequency is present in a particular function. The depicted function {{math|''f''&thinsp;(''t'') {{=}} cos(6π''t'') ''e''<sup>−π''t''<sup>2</sup></sup>}} oscillates at 3&nbsp;[[hertz|Hz]] (if {{mvar|t}} measures seconds) and tends quickly to 0. (The second factor in this equation is an [[Envelope (waves)|envelope function]] that shapes the continuous sinusoid into a short pulse. Its general form is a [[Gaussian function]]). This function was specially chosen to have a real Fourier transform that can be easily plotted. The first image contains its graph. In order to calculate <math>\hat{f}(3)</math> we must integrate {{math|''e''<sup>−2π''i''(3''t'')</sup>''f''&thinsp;(''t'')}}. The second image shows the plot of the real and imaginary parts of this function. The real part of the integrand is almost always positive, because when {{math|''f''&thinsp;(''t'')}} is negative, the real part of {{math|''e''<sup>−2π''i''(3''t'')</sup>}} is negative as well. Because they oscillate at the same rate, when {{math|''f''&thinsp;(''t'')}} is positive, so is the real part of {{math|''e''<sup>−2π''i''(3''t'')</sup>}}. The result is that when you integrate the real part of the integrand you get a relatively large number (in this case {{sfrac|1|2}}). On the other hand, when you try to measure a frequency that is not present, as in the case when we look at <math>\hat{f}(5)</math>, you see that both real and imaginary component of this function vary rapidly between positive and negative values, as plotted in the third image. Therefore, in this case, the integrand oscillates fast enough so that the integral is very small and the value for the Fourier transform for that frequency is nearly zero.

The general situation may be a bit more complicated than this, but this in spirit is how the Fourier transform measures how much of an individual frequency is present in a function {{math|''f''&thinsp;(''t'')}}.
<gallery widths="360px" heights="360px">
File:Function ocsillating at 3 hertz.svg|Original function showing oscillation 3&nbsp;Hz.
File:Onfreq.svg| Real and imaginary parts of integrand for Fourier transform at 3&nbsp;Hz
File:Offfreq.svg| Real and imaginary parts of integrand for Fourier transform at 5&nbsp;Hz
File:Fourier transform of oscillating function.svg| Magnitude of Fourier transform, with 3 and 5&nbsp;Hz labeled.
</gallery>

==Properties of the Fourier transform==
Here we assume {{math|''f''&thinsp;(''x'')}}, {{math|''g''(''x'')}} and {{math|''h''(''x'')}} are ''integrable functions'': [[Lebesgue-measurable]] on the real line satisfying:

:<math>\int_{-\infty}^\infty |f(x)| \, dx < \infty.</math>

We denote the Fourier transforms of these functions as {{math|''f̂''&thinsp;(''ξ'')}}, {{math|''ĝ''(''ξ'')}} and {{math|''ĥ''(''ξ'')}} respectively.

===Basic properties===
The Fourier transform has the following basic properties:<ref name="Pinsky-2002">{{harvnb|Pinsky|2002}}.</ref>

====Linearity====

: For any [[complex number]]s {{mvar|a}} and {{mvar|b}}, if {{math|''h''(''x'') {{=}} ''af''&nbsp;(''x'') + ''bg''(''x'')}}, then {{math|''ĥ''(''ξ'') {{=}} ''a'' · ''f̂''&thinsp;(''ξ'') + ''b'' · ''ĝ''(''ξ'')}}.

====Translation / time shifting====
[[File:Fourier transform - time shifted signal.gif|thumb|middle|right|upright=1.75|Animation showing the Fourier Transform of a time shifted signal. [Top] the original signal (orange), is continuously time shifted (blue). [Bottom] The resultant Fourier Transform of the time shifted signal. Note how the higher frequency components revolve in complex plane faster than the lower frequency components]]
: For any [[real number]] {{math|''x''<sub>0</sub>}}, if {{math|''h''(''x'') {{=}} ''f''&thinsp;(''x'' − ''x''<sub>0</sub>)}}, then {{math|''ĥ''(''ξ'') {{=}} ''e''<sup>−2π''ix''<sub>0</sub>''ξ''</sup> ''f̂''&thinsp;(''ξ'')}}.

====Modulation / frequency shifting====

: For any [[real number]] {{math|''ξ''<sub>0</sub>}}, if {{math|''h''(''x'') {{=}} ''e''<sup>2π''ixξ''<sub>0</sub></sup> ''f''&thinsp;(''x'')}}, then {{math|''ĥ''(''ξ'') {{=}} ''f̂''&thinsp;(''ξ'' − ''ξ''<sub>0</sub>)}}.

====Time scaling====

: For a non-zero [[real number]] {{mvar|a}}, if {{math|''h''(''x'') {{=}} ''f''&thinsp;(''ax'')}}, then 
:: <math>\hat{h}(\xi)=\frac{1}{|a|}\hat{f}\left(\frac{\xi}{a}\right).</math>
:The case {{math|''a'' {{=}} −1}} leads to the ''time-reversal'' property, which states: if {{math|''h''(''x'') {{=}} ''f''&thinsp;(−''x'')}}, then {{math|''ĥ''(''ξ'') {{=}} ''f̂''&thinsp;(−''ξ'')}}.

====[[complex conjugate|Conjugation]]====

: If {{math|''h''(''x'') {{=}} {{overline|''f''&thinsp;(''x'')}}}}, then 
:: <math>\hat{h}(\xi) = \overline{\hat{f}(-\xi)}.</math>
: In particular, if {{mvar|f}} is real, then one has the ''reality condition'' 
:: <math>\hat{f}(-\xi)=\overline{\hat{f}(\xi)},</math>
: that is, {{math|''f̂''}} is a [[Hermitian function]]. And if {{mvar|f}} is purely imaginary, then 
:: <math>\hat{f}(-\xi)=-\overline{\hat{f}(\xi)}.</math>

====Real and imaginary part in time====
* If <math>h(x) = \Re{(f(x))}</math>, then <math>\hat{h}(\xi) = \frac{1}{2}(\hat{f}(\xi) + \overline{\hat{f}(-\xi)})</math>.
* If <math>h(x) = \Im{(f(x))}</math>, then <math>\hat{h}(\xi) = \frac{1}{2i}(\hat{f}(\xi) - \overline{\hat{f}(-\xi)})</math>.

====Integration====

: Substituting {{math|''ξ'' {{=}} 0}} in the definition, we obtain
:: <math>\hat{f}(0) = \int_{-\infty}^{\infty} f(x)\,dx.</math>
: That is, the evaluation of the Fourier transform at the origin ({{math|''ξ'' {{=}} 0}}) equals the integral of {{mvar|f}} over all its domain.

===Invertibility and periodicity===
{{further|Fourier inversion theorem|Fractional Fourier transform}}
Under suitable conditions on the function {{mvar|f}}, it can be recovered from its Fourier transform <math>\hat{f}</math>. Indeed, denoting the Fourier transform operator by {{mathcal|F}}, so {{math|{{mathcal|F}}(&thinsp;''f''&thinsp;) :{{=}} ''f̂''}}, then for suitable functions, applying the Fourier transform twice simply flips the function: {{math|{{mathcal|F}}{{isup|2}}(&thinsp;''f''&thinsp;)(''x'') {{=}} ''f''&thinsp;(−''x'')}}, which can be interpreted as "reversing time". Since reversing time is two-periodic, applying this twice yields {{math|{{mathcal|F}}{{isup|4}}(&thinsp;''f''&thinsp;) {{=}} ''f''}}, so the Fourier transform operator is four-periodic, and similarly the inverse Fourier transform can be obtained by applying the Fourier transform three times: {{math|{{mathcal|F}}{{isup|3}}(&thinsp;''f̂''&thinsp;) {{=}} ''f''}}. In particular the Fourier transform is invertible (under suitable conditions).

More precisely, defining the ''parity operator'' {{mathcal|P}} that inverts time, {{math|{{mathcal|P}}[&thinsp;''f''&thinsp;] : ''t'' ↦ ''f''&thinsp;(−''t'')}}:
:<math>\begin{align}
  \mathcal{F}^0 &= \mathrm{Id}, \quad
  \mathcal{F}^1  = \mathcal{F}, \\
  \mathcal{F}^2 &= \mathcal{P}, \quad
  \mathcal{F}^3 = \mathcal{F}^{-1} = \mathcal{P} \circ \mathcal{F} = \mathcal{F} \circ \mathcal{P}, \\
  \mathcal{F}^4  &= \mathrm{Id}
\end{align}</math>
These equalities of operators require careful definition of the space of functions in question, defining equality of functions (equality at every point? equality [[almost everywhere]]?) and defining equality of operators – that is, defining the topology on the function space and operator space in question. These are not true for all functions, but are true under various conditions, which are the content of the various forms of the [[Fourier inversion theorem]].

This fourfold periodicity of the Fourier transform is similar to a rotation of the plane by 90°, particularly as the two-fold iteration yields a reversal, and in fact this analogy can be made precise. While the Fourier transform can simply be interpreted as switching the time domain and the frequency domain, with the inverse Fourier transform switching them back, more geometrically it can be interpreted as a rotation by 90° in the [[time–frequency domain]] (considering time as the {{mvar|x}}-axis and frequency as the {{mvar|y}}-axis), and the Fourier transform can be generalized to the [[fractional Fourier transform]], which involves rotations by other angles. This can be further generalized to [[linear canonical transformation]]s, which can be visualized as the action of the [[special linear group]] {{math|[[SL2(R)|SL<sub>2</sub>('''ℝ''')]]}} on the time–frequency plane, with the preserved symplectic form corresponding to the [[#Uncertainty principle|uncertainty principle]], below. This approach is particularly studied in [[signal processing]], under [[time–frequency analysis]].

===Units and duality===

In mathematics, one often does not think of any units as being attached to the two variables {{mvar|t}} and {{mvar|ξ}}. <!--If one identifies these two copies of the real line, then one can regard the Fourier transformation as a linear mapping from {{math|''L''<sup>2</sup>('''ℝ''')}} to itself.--> But in physical applications, {{mvar|ξ}} must have inverse units to the units of {{mvar|t}}. For example, if {{mvar|t}} is measured in seconds, {{mvar|ξ}}
should be in cycles per second for the formulas here to be valid. If the scale of {{mvar|t}} is changed and {{mvar|t}} is measured in units of 2{{pi}} seconds, then either {{mvar|ξ}} must be in the so-called "[[angular frequency]]", or one must insert some constant scale factor into some of the formulas. If {{mvar|t}} is measured in units of length, then {{mvar|ξ}} must be in inverse length, e.g., [[Wavenumber|wavenumbers]]. That is to say, there are two copies of the real line: one measured in one set of units, where {{mvar|t}} ranges, and the other in inverse units to the units of {{mvar|t}}, and which is the range of {{mvar|ξ}}. So these are two distinct copies of the real line, and cannot be identified with each other. Therefore, the Fourier transform goes from one space of functions to a different space of functions: functions which have a different domain of definition.

In general, {{mvar|ξ}} must always be taken to be a [[linear form]] on the space of {{mvar|t}}s, which is to say that the second real line is the [[dual space]] of the first real line. See the article on [[linear algebra]] for a more formal explanation and for more details. This point of view becomes essential in generalisations of the Fourier transform to general [[symmetry groups]], including the case of Fourier series.

That there is no one preferred way (often, one says "no canonical way") to compare the two copies of the real line which are involved in the Fourier transform—fixing the units on one line does not force the scale of the units on the other line—is the reason for the plethora of rival conventions on the definition of the Fourier transform. The various definitions resulting from different choices of units differ by various constants. If the units of {{mvar|t}} are in seconds but the units of {{mvar|ξ}} are in angular frequency, then the angular frequency variable is often denoted by one or another Greek letter, for example, {{math|''ω'' {{=}} 2π''ξ''}} is quite common. Thus (writing {{math|''x̂''<sub>1</sub>}} for the alternative definition and {{math|''x̂''}} for the definition adopted in this article)

:<math>\hat x_1(\omega) = \hat x \left(\frac{\omega}{2\pi}\right) = \int_{-\infty}^\infty x(t) e^{-i\omega t} \, dt</math>

as before, but the corresponding alternative inversion formula would then have to be

:<math> x(t) = \frac{1}{2\pi}\int_{-\infty}^\infty \hat x_1(\omega) e^{it\omega} \, d\omega .</math>

To have something involving angular frequency but with greater symmetry between the Fourier transform and the inversion formula, one very often sees still another alternative definition of the Fourier transform, with a factor of {{sqrt|2{{pi}}}}, thus

:<math>\hat x_2 (\omega) = \frac 1 {\sqrt{2\pi}} \int_{-\infty}^\infty x(t) e^{-i\omega t} \, dt ,</math>
and the corresponding inversion formula then has to be

:<math> x(t) = \frac{1}{\sqrt{2\pi}}\int_{-\infty}^\infty \hat x_2(\omega) e^{it\omega} \, d\omega .</math>

In some unusual conventions, such as those employed by the FourierTransform command of the [[Wolfram Language]], the Fourier transform has {{mvar|i}} in the exponent instead of {{math|−''i''}}, and vice versa for the inversion formula.  Many of the identities involving the Fourier transform remain valid in those conventions, provided all terms that explicitly involve {{math|''i''}} have it replaced by {{math|&minus;''i''}}.

For example, in probability theory, the characteristic function {{mvar|ϕ}} of the probability density function {{mvar|f}} of a random variable {{mvar|X}} of continuous type is defined without a negative sign in the exponential, and since the units of {{mvar|x}} are ignored, there is no 2{{pi}} either:

:<math>\phi (\lambda) = \int_{-\infty}^\infty f(x) e^{i\lambda x} \,dx.</math>

(In probability theory, and in mathematical statistics, the use of the Fourier—Stieltjes transform is preferred, because so many random variables are not of continuous type, and do not possess a density function, and one must treat not functions but [[Distribution (mathematics)|distributions]], i.e., measures which possess "atoms".)

From the higher point of view of [[Character theory|group characters]], which is much more abstract, all these arbitrary choices disappear, as will be explained in the later section of this article, which treats the notion of the Fourier transform of a function on a [[Locally compact abelian group|locally compact Abelian group]].

===Uniform continuity and the Riemann–Lebesgue lemma===
[[File:Rectangular function.svg|thumb|The [[rectangular function]] is [[Lebesgue integrable]].]]
[[File:Sinc function (normalized).svg|thumb|The [[sinc function]], which is the Fourier transform of the rectangular function, is bounded and continuous, but not Lebesgue integrable.]]
The Fourier transform may be defined in some cases for non-integrable functions, but the Fourier transforms of integrable functions have several strong properties.

The Fourier transform {{math|''f̂''}} of any integrable function {{mvar|f}} is [[uniformly continuous]] and<ref name="Katznelson-1976">{{harvnb|Katznelson|1976}}.</ref>
:<math>\left\|\hat{f}\right\|_\infty \leq \left\|f\right\|_1</math>

By the ''[[Riemann–Lebesgue lemma]]'',<ref name="Stein-Weiss-1971">{{harvnb|Stein|Weiss|1971}}.</ref>
:<math>\hat{f}(\xi) \to 0\text{ as }|\xi| \to \infty.</math>

However, <math>\hat{f}</math> need not be integrable. For example, the Fourier transform of the [[rectangular function]], which is integrable, is the [[sinc function]], which is not [[Lebesgue integrable]], because its [[improper integral]]s behave analogously to the [[alternating harmonic series]], in converging to a sum without being [[absolutely convergent]].

It is not generally possible to write the ''inverse transform'' as a [[Lebesgue integral]]. However, when both {{mvar|f}} and <math>\hat{f}</math> are integrable, the inverse equality

:<math>f(x) = \int_{-\infty}^\infty \hat f(\xi) e^{2 i \pi x \xi} \, d\xi</math>

holds [[almost everywhere]]. That is, the Fourier transform is [[injective]] on {{math|[[Lp space|''L''<sup>1</sup>('''ℝ''')]]}}. (But if {{mvar|f}} is continuous, then equality holds for every {{mvar|x}}.)

===Plancherel theorem and Parseval's theorem===
Let {{math|''f''&thinsp;(''x'')}} and {{math|''g''(''x'')}} be integrable, and let {{math|''f̂''&thinsp;(''ξ'')}} and {{math|''ĝ''(''ξ'')}} be their Fourier transforms. If ''f''&thinsp;(''x'') and ''g''(''x'') are also [[square-integrable]], then the Parseval formula follows:<ref>{{harvnb|Rudin|1987|p=187}}.</ref>

: <math>\int_{-\infty}^{\infty} f(x) \overline{g(x)} \,dx = \int_{-\infty}^\infty \hat{f}(\xi) \overline{\hat{g}(\xi)} \,d\xi,</math>

where the bar denotes [[complex conjugation]].

The [[Plancherel theorem]], which follows from the above, states that<ref>{{harvnb|Rudin|1987|p=186}}.</ref>

:<math>\int_{-\infty}^\infty \left| f(x) \right|^2\,dx = \int_{-\infty}^\infty \left| \hat{f}(\xi) \right|^2\,d\xi. </math>

Plancherel's theorem makes it possible to extend the Fourier transform, by a continuity argument, to a [[unitary operator]] on {{math|''L''<sup>2</sup>('''ℝ''')}}. On {{math|''L''<sup>1</sup>('''ℝ''') ∩ ''L''<sup>2</sup>('''ℝ''')}}, this extension agrees with original Fourier transform defined on {{math|''L''<sup>1</sup>('''ℝ''')}}, thus enlarging the domain of the Fourier transform to {{math|''L''<sup>1</sup>('''ℝ''') + ''L''<sup>2</sup>('''ℝ''')}} (and consequently to {{math|{{math|''L''{{isup|''p''}}}}('''ℝ''')}} for {{math|1 ≤ ''p'' ≤ 2}}). Plancherel's theorem has the interpretation in the sciences that the Fourier transform preserves the [[energy]] of the original quantity. The terminology of these formulas is not quite standardised. Parseval's theorem was proved only for Fourier series, and was first proved by Lyapunov. But Parseval's formula makes sense for the Fourier transform as well, and so even though in the context of the Fourier transform it was proved by Plancherel, it is still often referred to as Parseval's formula, or Parseval's relation, or even Parseval's theorem.

See [[Pontryagin duality]] for a general formulation of this concept in the context of locally compact abelian groups.

===Poisson summation formula===
{{Main|Poisson summation formula}}

The Poisson summation formula (PSF) is an equation that relates the [[Fourier series]] coefficients of the [[periodic summation]] of a function to values of the function's continuous Fourier transform. The Poisson summation formula says that for sufficiently regular functions {{mvar|f}},

:<math> \sum_n \hat f(n) = \sum_n f (n).</math>

It has a variety of useful forms that are derived from the basic one by application of the Fourier transform's scaling and time-shifting properties. The formula has applications in engineering, physics, and [[number theory]]. The frequency-domain dual of the standard Poisson summation formula is also called the [[discrete-time Fourier transform]].

Poisson summation is generally associated with the physics of periodic media, such as heat conduction on a circle. The fundamental solution of the heat equation on a circle is called a [[theta function]]. It is used in [[number theory]] to prove the transformation properties of theta functions, which turn out to be a type of [[modular form]], and it is connected more generally to the theory of [[automorphic form]]s where it appears on one side of the [[Selberg trace formula]].

===Differentiation===
Suppose {{math|''f''&thinsp;(''x'')}} is an absolutely continuous differentiable function, and both {{math|''f''}} and its derivative {{math|''f&thinsp;′''}} are integrable. Then the Fourier transform of the derivative is given by
:<math>\widehat{f'\;}(\xi)=2\pi i\xi\hat{f}(\xi).</math>
More generally, the Fourier transformation of the {{mvar|n}}th derivative {{math|''f''{{isup|(''n'')}}}} is given by
:<math>\widehat{f^{(n)}}(\xi)=(2\pi i\xi)^n\hat{f}(\xi).</math>

By applying the Fourier transform and using these formulas, some [[ordinary differential equation]]s can be transformed into algebraic equations, which are much easier to solve. These formulas also give rise to the rule of thumb "{{math|''f''&thinsp;(''x'')}} is smooth [[if and only if]] {{math|''f̂''&thinsp;(''ξ'')}} quickly falls to 0 for {{math|{{abs|''ξ''}} → ∞}}." By using the analogous rules for the inverse Fourier transform, one can also say "{{math|''f''&thinsp;(''x'')}} quickly falls to 0 for {{math|{{abs|''x''}} → ∞}} if and only if {{math|''f̂''&thinsp;(''ξ'')}} is smooth."

===Convolution theorem===
{{Main|Convolution theorem}}

The Fourier transform translates between [[convolution]] and multiplication of functions. If {{math|''f''&thinsp;(''x'')}} and {{math|''g''(''x'')}} are integrable functions with Fourier transforms {{math|''f̂''&thinsp;(''ξ'')}} and {{math|''ĝ''(''ξ'')}} respectively, then the Fourier transform of the convolution is given by the product of the Fourier transforms {{math|''f̂''&thinsp;(''ξ'')}} and {{math|''ĝ''(''ξ'')}} (under other conventions for the definition of the Fourier transform a constant factor may appear).

This means that if:

:<math>h(x) = (f*g)(x) = \int_{-\infty}^\infty f(y)g(x - y)\,dy,</math>

where {{math|∗}} denotes the convolution operation, then:

:<math>\hat{h}(\xi) = \hat{f}(\xi)\cdot \hat{g}(\xi).</math>

In [[LTI system theory|linear time invariant (LTI) system theory]], it is common to interpret {{math|''g''(''x'')}} as the [[impulse response]] of an LTI system with input {{math|''f''&thinsp;(''x'')}} and output {{math|''h''(''x'')}}, since substituting the [[Dirac delta function|unit impulse]] for {{math|''f''&thinsp;(''x'')}} yields {{math|''h''(''x'') {{=}} ''g''(''x'')}}. In this case, {{math|''ĝ''(''ξ'')}} represents the [[frequency response]] of the system.

Conversely, if {{math|''f''&thinsp;(''x'')}} can be decomposed as the product of two square integrable functions {{math|''p''(''x'')}} and {{math|''q''(''x'')}}, then the Fourier transform of {{math|''f''&thinsp;(''x'')}} is given by the convolution of the respective Fourier transforms {{math|''p̂''(''ξ'')}} and {{math|''q̂''(''ξ'')}}.

===Cross-correlation theorem===
{{Main|Cross-correlation}}

In an analogous manner, it can be shown that if {{math|''h''(''x'')}} is the [[cross-correlation]] of {{math|''f''&thinsp;(''x'')}} and {{math|''g''(''x'')}}:

:<math>h(x) = (f \star g)(x) = \int_{-\infty}^\infty \overline{f(y)}g(x + y)\,dy</math>

then the Fourier transform of {{math|''h''(''x'')}} is:

:<math>\hat{h}(\xi) = \overline{\hat{f}(\xi)} \cdot \hat{g}(\xi).</math>

As a special case, the [[autocorrelation]] of function {{math|''f''&thinsp;(''x'')}} is:

:<math>h(x) = (f \star f)(x) = \int_{-\infty}^\infty \overline{f(y)}f(x + y)\,dy</math>

for which

:<math>\hat{h}(\xi) = \overline{\hat{f}(\xi)}\hat{f}(\xi) = \left|\hat{f}(\xi)\right|^2.</math>

===Eigenfunctions===
One important choice of an orthonormal basis for {{math|[[Square-integrable function|''L''<sup>2</sup>('''ℝ''')]]}} is given by the Hermite functions

: <math>\psi_n(x) = \frac{\sqrt[4]{2}}{\sqrt{n!}} e^{-\pi x^2}\mathrm{He}_n\left(2x\sqrt{\pi}\right),</math>

where {{math|He<sub>''n''</sub>(''x'')}} are the "probabilist's" [[Hermite polynomial]]s, defined as

: <math>\mathrm{He}_n(x) = (-1)^n e^{\frac{x^2}{2}}\left(\frac{d}{dx}\right)^n e^{-\frac{x^2}{2}}</math>

Under this convention for the Fourier transform, we have that

: <math> \hat\psi_n(\xi) = (-i)^n \psi_n(\xi) </math>.

In other words, the Hermite functions form a complete [[orthonormal]] system of [[eigenfunctions]] for the Fourier transform on {{math|''L''<sup>2</sup>('''ℝ''')}}.<ref name="Pinsky-2002" /> However, this choice of eigenfunctions is not unique. There are only four different [[eigenvalue]]s of the Fourier transform (±1 and ±{{mvar|i}}) and any linear combination of eigenfunctions with the same eigenvalue gives another eigenfunction. As a consequence of this, it is possible to decompose {{math|''L''<sup>2</sup>('''ℝ''')}} as a direct sum of four spaces {{math|''H''<sub>0</sub>}}, {{math|''H''<sub>1</sub>}}, {{math|''H''<sub>2</sub>}}, and {{math|''H''<sub>3</sub>}} where the Fourier transform acts on {{math|He<sub>''k''</sub>}} simply by multiplication by {{math|''i''<sup>''k''</sup>}}.

Since the complete set of Hermite functions provides a resolution of the identity, the Fourier transform can be represented by such a sum of terms weighted by the above eigenvalues, and these sums can be explicitly summed. This approach to define the Fourier transform was first done by [[Norbert Wiener]].<ref name="Duoandikoetxea-2001">{{harvnb|Duoandikoetxea|2001}}.</ref> Among other properties, Hermite functions decrease exponentially fast in both frequency and time domains, and they are thus used to define a generalization of the Fourier transform, namely the [[fractional Fourier transform]] used in time-frequency analysis.<ref name="Boashash-2003">{{harvnb|Boashash|2003}}.</ref> In [[physics]], this transform was introduced by [[Edward Condon]].<ref>{{harvnb|Condon|1937}}.</ref>

===Connection with the Heisenberg group===
The [[Heisenberg group]] is a certain [[group (mathematics)|group]] of [[unitary operator]]s on the [[Hilbert space]] {{math|''L''<sup>2</sup>('''ℝ''')}} of square integrable complex valued functions {{mvar|f}} on the real line, generated by the translations {{math|(''T<sub>y</sub> f''&thinsp;)(''x'') {{=}} ''f''&nbsp;(''x'' + ''y'')}} and multiplication by {{math|''e''<sup>2π''ixξ''</sup>}}, {{math|(''M<sub>ξ</sub> f''&thinsp;)(''x'') {{=}} ''e''<sup>2π''ixξ''</sup> ''f''&nbsp;(''x'')}}. These operators do not commute, as their (group) commutator is
:<math>\left(M^{-1}_\xi T^{-1}_yM_\xi T_yf\right)(x) = e^{2\pi i y\xi}f(x)</math>
which is multiplication by the constant (independent of {{mvar|x}}) {{math|''e''<sup>2π''iyξ''</sup> ∈ ''U''(1)}} (the [[circle group]] of unit modulus complex numbers). As an abstract group, the Heisenberg group is the three-dimensional [[Lie group]] of triples {{math|(''x'', ''ξ'', ''z'') ∈ '''ℝ'''<sup>2</sup> × ''U''(1)}}, with the group law
:<math>\left(x_1, \xi_1, t_1\right) \cdot \left(x_2, \xi_2, t_2\right) = \left(x_1 + x_2, \xi_1 + \xi_2, t_1 t_2 e^{2\pi i \left(x_1\xi_1 + x_2\xi_2 + x_1\xi_2\right)}\right).</math>

Denote the Heisenberg group by {{math|''H''<sub>1</sub>}}. The above procedure describes not only the group structure, but also a standard [[unitary representation]] of {{math|''H''<sub>1</sub>}} on a Hilbert space, which we denote by {{math|''ρ'' : ''H''<sub>1</sub> → ''B''(''L''<sup>2</sup>('''ℝ'''))}}. Define the linear automorphism of {{math|'''ℝ'''<sup>2</sup>}} by
:<math>J \begin{pmatrix}
   x \\
  \xi
\end{pmatrix} = \begin{pmatrix}
  -\xi \\
    x
\end{pmatrix}</math>

so that {{math|''J''{{isup|2}} {{=}} −''I''}}. This {{mvar|J}} can be extended to a unique automorphism of {{math|''H''<sub>1</sub>}}:
:<math>j\left(x, \xi, t\right) = \left(-\xi, x, te^{-2\pi i x\xi}\right).</math>

According to the [[Stone–von Neumann theorem]], the unitary representations {{mvar|ρ}} and {{math|''ρ'' ∘ ''j''}} are unitarily equivalent, so there is a unique intertwiner {{math|''W'' ∈ ''U''(''L''<sup>2</sup>('''ℝ'''))}} such that
:<math>\rho \circ j = W \rho W^*.</math>
This operator {{mvar|W}} is the Fourier transform.

Many of the standard properties of the Fourier transform are immediate consequences of this more general framework.<ref>{{harvnb|Howe|1980}}.</ref> For example, the square of the Fourier transform, {{math|''W''{{isup|2}}}}, is an intertwiner associated with {{math|''J''{{isup|2}} {{=}} −''I''}}, and so we have {{math|(''W''{{isup|2}}''f''&thinsp;)(''x'') {{=}} ''f''&nbsp;(−''x'')}} is the reflection of the original function {{mvar|f}}.

== Complex domain ==

The [[integral]] for the Fourier transform
: <math> \hat f (\xi) = \int _{-\infty}^\infty e^{-2\pi i \xi t} f(t) \, dt </math>
can be studied for [[complex number|complex]] values of its argument {{mvar|ξ}}. Depending on the properties of {{mvar|f}}, this might not converge off the real axis at all, or it might converge to a [[complex analysis|complex]] [[analytic function]] for all values of {{math|''ξ'' {{=}} ''σ'' + ''iτ''}}, or something in between.<ref>{{harvnb|Paley|Wiener|1934}}.</ref>

The [[Paley–Wiener theorem]] says that {{mvar|f}} is smooth (i.e., {{mvar|n}}-times differentiable for all positive integers {{mvar|n}}) and compactly supported if and only if {{math|''f̂''&nbsp;(''σ'' + ''iτ'')}} is a [[holomorphic function]] for which there exists a [[constant (mathematics)|constant]] {{math|''a'' > 0}} such that for any [[integer]] {{math|''n'' ≥ 0}},
: <math> \left\vert \xi ^n \hat f(\xi) \right\vert \leq C e^{a\vert\tau\vert} </math>
for some constant {{mvar|C}}. (In this case, {{mvar|f}} is supported on {{math|[−''a'', ''a'']}}.) This can be expressed by saying that {{math|''f̂''}} is an [[entire function]] which is [[rapidly decreasing]] in {{mvar|σ}} (for fixed {{mvar|τ}}) and of exponential growth in {{mvar|τ}} (uniformly in {{mvar|σ}}).<ref>{{harvnb|Gelfand|Vilenkin|1964}}.</ref>

(If {{mvar|f}} is not smooth, but only {{math|''L''<sup>2</sup>}}, the statement still holds provided {{math|''n'' {{=}} 0}}.<ref>{{harvnb|Kirillov|Gvishiani|1982}}.</ref>) The space of such functions of a [[complex analysis|complex variable]] is called the Paley—Wiener space. This theorem has been generalised to semisimple [[Lie group]]s.<ref>{{harvnb|Clozel|Delorme|1985|pp=331–333}}.</ref>

If {{mvar|f}} is supported on the half-line {{math|''t'' ≥ 0}}, then {{mvar|f}} is said to be "causal" because the [[impulse response function]] of a physically realisable [[Filter (mathematics)|filter]] must have this property, as no effect can precede its cause. [[Raymond Paley|Paley]] and Wiener showed that then {{math|''f̂''}} extends to a [[holomorphic function]] on the complex lower half-plane {{math|''τ'' < 0}} which tends to zero as {{mvar|τ}} goes to infinity.<ref>{{harvnb|de Groot|Mazur|1984|p=146}}.</ref> The converse is false and it is not known how to characterise the Fourier transform of a causal function.<ref>{{harvnb|Champeney|1987|p=80}}.</ref>

=== Laplace transform ===

The Fourier transform {{math|''f̂''&thinsp;(''ξ'')}} is related to the [[Laplace transform]] {{math|''F''(''s'')}}, which is also used for the solution of [[differential equation]]s and the analysis of [[Filter (mathematics)|filter]]s.

It may happen that a function {{mvar|f}} for which the Fourier integral does not converge on the real axis at all, nevertheless has a complex Fourier transform defined in some region of the [[complex plane]].

For example, if {{math|''f''&thinsp;(''t'')}} is of exponential growth, i.e.,
: <math> \vert f(t) \vert < C e^{a\vert t\vert} </math>
for some constants {{math|''C'', ''a'' ≥ 0}}, then<ref name="Kolmogorov-Fomin-1999">{{harvnb|Kolmogorov|Fomin|1999}}.</ref>
: <math> \hat f (i\tau) = \int _{-\infty}^\infty e^{ 2\pi \tau t} f(t) \, dt, </math>
convergent for all {{math|2π''τ'' < −''a''}}, is the [[two-sided Laplace transform]] of {{mvar|f}}.

The more usual version ("one-sided") of the Laplace transform is
: <math> F(s) = \int_0^\infty f(t) e^{-st} \, dt.</math>

If {{mvar|f}} is also causal, then
: <math> \hat f(i\tau) = F(-2\pi\tau).</math>

Thus, extending the Fourier transform to the complex domain means it includes the Laplace transform as a special case—the case of causal functions—but with the change of variable {{math|''s'' {{=}} 2π''iξ''}}.

=== Inversion ===

If {{math|''f̂''}} is complex analytic for {{math|''a'' ≤ ''τ'' ≤ ''b''}}, then

: <math> \int _{-\infty}^\infty \hat f (\sigma + ia) e^{ 2\pi i \xi t} \, d\sigma = \int _{-\infty}^\infty \hat f (\sigma + ib) e^{ 2\pi i \xi t} \, d\sigma </math>
by [[Cauchy's integral theorem]]. Therefore, the Fourier inversion formula can use integration along different lines, parallel to the real axis.<ref>{{harvnb|Wiener|1949}}.</ref>

Theorem: If {{math|''f''&thinsp;(''t'') {{=}} 0}} for {{math|''t'' < 0}}, and {{math|{{abs|''f''&thinsp;(''t'')}} < ''Ce''<sup>''a''{{abs|''t''}}</sup>}} for some constants {{math|''C'', ''a'' > 0}}, then
: <math> f(t) = \int_{-\infty}^\infty \hat f(\sigma + i\tau) e^{2 \pi i\xi t} \, d\sigma,</math>
for any {{math|''τ'' < −{{sfrac|''a''|2π}}}}.

This theorem implies the [[inverse Laplace transform|Mellin inversion formula]] for the Laplace transformation,<ref name="Kolmogorov-Fomin-1999" />
: <math> f(t) = \frac 1 {2\pi i} \int_{b-i\infty}^{b+i\infty} F(s) e^{st}\, ds</math>
for any {{math|''b'' > ''a''}}, where {{math|''F''(''s'')}} is the Laplace transform of {{math|''f''&thinsp;(''t'')}}.

The hypotheses can be weakened, as in the results of Carleman and Hunt, to {{math|''f''&thinsp;(''t'') ''e''<sup>−''at''</sup>}} being {{math|''L''<sup>1</sup>}}, provided that {{mvar|f}} is of bounded variation in a closed neighborhood of {{mvar|t}} (cf. [[Dirichlet-Dini theorem]]), the value of {{mvar|f}} at {{mvar|t}} is taken to be the [[arithmetic mean]] of the left and right limits, and provided that the integrals are taken in the sense of Cauchy principal values.<ref>{{harvnb|Champeney|1987|p=63}}.</ref>

{{math|''L''<sup>2</sup>}} versions of these inversion formulas are also available.<ref>{{harvnb|Widder|Wiener|1938|p=537}}.</ref>

==Fourier transform on Euclidean space==
The Fourier transform can be defined in any arbitrary number of dimensions {{mvar|n}}. As with the one-dimensional case, there are many conventions. For an integrable function {{math|''f''&thinsp;('''x''')}}, this article takes the definition:

:<math>\hat{f}(\boldsymbol{\xi}) = \mathcal{F}(f)(\boldsymbol{\xi}) = \int_{\R^n} f(\mathbf{x}) e^{-2\pi i \mathbf{x}\cdot\boldsymbol{\xi}} \, d\mathbf{x}</math>

where {{math|'''x'''}} and {{math|'''ξ'''}} are {{mvar|n}}-dimensional [[vector (mathematics)|vectors]], and {{math|'''x''' · '''ξ'''}} is the [[dot product]] of the vectors. The dot product is sometimes written as {{math|⟨'''x''', '''ξ'''⟩}}.

All of the basic properties listed above hold for the {{mvar|n}}-dimensional Fourier transform, as do Plancherel's and Parseval's theorem. When the function is integrable, the Fourier transform is still uniformly continuous and the [[Riemann–Lebesgue lemma]] holds.<ref name="Stein-Weiss-1971" />

===Uncertainty principle===
{{details|Gabor limit}}
Generally speaking, the more concentrated {{math|''f''&thinsp;(''x'')}} is, the more spread out its Fourier transform {{math|''f̂''&thinsp;(''ξ'')}} must be. In particular, the scaling property of the Fourier transform may be seen as saying: if we squeeze a function in {{mvar|x}}, its Fourier transform stretches out in {{mvar|ξ}}. It is not possible to arbitrarily concentrate both a function and its Fourier transform.

The trade-off between the compaction of a function and its Fourier transform can be formalized in the form of an [[uncertainty principle]] by viewing a function and its Fourier transform as [[conjugate variables]] with respect to the [[symplectic form]] on the [[time–frequency representation|time–frequency domain]]: from the point of view of the [[linear canonical transformation]], the Fourier transform is rotation by 90° in the time–frequency domain, and preserves the [[Symplectic vector space|symplectic form]].

Suppose {{math|''f''&thinsp;(''x'')}} is an integrable and [[square-integrable]] function. Without loss of generality, assume that {{math|''f''&thinsp;(''x'')}} is normalized:

:<math>\int_{-\infty}^\infty |f(x)|^2 \,dx=1.</math>

It follows from the [[Plancherel theorem]] that {{math|''f̂''&thinsp;(''ξ'')}} is also normalized.

The spread around {{math|''x'' {{=}} 0}} may be measured by the ''dispersion about zero''<ref>{{harvnb|Pinsky|2002|p=131}}.</ref> defined by

:<math>D_0(f)=\int_{-\infty}^\infty x^2|f(x)|^2\,dx.</math>

In probability terms, this is the [[Moment (mathematics)|second moment]] of {{math|{{abs|''f''&thinsp;(''x'')}}<sup>2</sup>}} about zero.

The uncertainty principle states that, if {{math|''f''&thinsp;(''x'')}} is absolutely continuous and the functions {{math|''x''·''f''&thinsp;(''x'')}} and {{math|''f''&thinsp;′(''x'')}} are square integrable, then<ref name="Pinsky-2002" />

:<math>D_0(f)D_0\left(\hat{f}\right) \geq \frac{1}{16\pi^2}</math>.

The equality is attained only in the case

:<math>\begin{align} f(x) &= C_1 \, e^{-\pi \frac{x^2}{\sigma^2} }\\
\therefore \hat{f}(\xi) &= \sigma C_1 \, e^{-\pi\sigma^2\xi^2} \end{align} </math>

where {{math|''σ'' > 0}} is arbitrary and {{math|''C''<sub>1</sub> {{=}} {{sfrac|{{radic|2|4}}|{{sqrt|''σ''}}}}}} so that {{mvar|f}} is {{math|''L''<sup>2</sup>}}-normalized.<ref name="Pinsky-2002" /> In other words, where {{mvar|f}} is a (normalized) [[Gaussian function]] with variance {{math|''σ''<sup>2</sup>}}, centered at zero, and its Fourier transform is a Gaussian function with variance {{math|''σ''<sup>−2</sup>}}.

In fact, this inequality implies that:

: <math>\left(\int_{-\infty}^\infty (x-x_0)^2|f(x)|^2\,dx\right)\left(\int_{-\infty}^\infty(\xi-\xi_0)^2\left|\hat{f}(\xi)\right|^2\,d\xi\right)\geq \frac{1}{16\pi^2}</math>

for any {{math|''x''<sub>0</sub>}}, {{math|''ξ''<sub>0</sub> ∈ '''ℝ'''}}.<ref name="Stein-Shakarchi-2003" />

In [[quantum mechanics]], the [[momentum]] and position [[wave function]]s are Fourier transform pairs, to within a factor of [[Planck's constant]]. With this constant properly taken into account, the inequality above becomes the statement of the [[Heisenberg uncertainty principle]].<ref>{{harvnb|Stein|Shakarchi|2003|p=158}}.</ref>

A stronger uncertainty principle is the [[Hirschman uncertainty|Hirschman uncertainty principle]], which is expressed as:

:<math>H\left(\left|f\right|^2\right)+H\left(\left|\hat{f}\right|^2\right)\ge \log\left(\frac{e}{2}\right)</math>

where {{math|''H''(''p'')}} is the [[differential entropy]] of the [[probability density function]] {{math|''p''(''x'')}}:

:<math>H(p) = -\int_{-\infty}^\infty p(x)\log\bigl(p(x)\bigr) \, dx</math>

where the logarithms may be in any base that is consistent. The equality is attained for a Gaussian, as in the previous case.

===Sine and cosine transforms===
{{main|Sine and cosine transforms}}

Fourier's original formulation of the transform did not use complex numbers, but rather sines and cosines. Statisticians and others still use this form. An absolutely integrable function {{mvar|f}} for which Fourier inversion holds good can be expanded in terms of genuine frequencies (avoiding negative frequencies, which are sometimes considered hard to interpret physically<ref>{{harvnb|Chatfield|2004|p=113}}.</ref>) {{mvar|λ}} by

:<math>f(t) = \int_0^\infty \bigl( a(\lambda ) \cos( 2\pi \lambda t) + b(\lambda ) \sin( 2\pi \lambda t)\bigr) \, d\lambda.</math>

This is called an expansion as a trigonometric integral, or a Fourier integral expansion. The coefficient functions {{mvar|a}} and {{mvar|b}} can be found by using variants of the Fourier cosine transform and the Fourier sine transform (the normalisations are, again, not standardised):

:<math> a (\lambda) = 2\int_{-\infty}^\infty f(t) \cos(2\pi\lambda t) \, dt</math>

and

:<math> b (\lambda) = 2\int_{-\infty}^\infty f(t) \sin(2\pi\lambda t) \, dt. </math>

Older literature refers to the two transform functions, the Fourier cosine transform, {{mvar|a}}, and the Fourier sine transform, {{mvar|b}}.

The function {{mvar|f}} can be recovered from the sine and cosine transform using

:<math> f(t) = 2\int_0 ^{\infty} \int_{-\infty}^{\infty} f(\tau) \cos\bigl( 2\pi \lambda(\tau-t)\bigr) \, d\tau \, d\lambda.</math>

together with trigonometric identities. This is referred to as Fourier's integral formula.<ref name="Kolmogorov-Fomin-1999" /><ref>{{harvnb|Fourier|1822|p=441}}.</ref><ref>{{harvnb|Poincaré|1895|p=102}}.</ref><ref>{{harvnb|Whittaker|Watson|1927|p=188}}.</ref>

===Spherical harmonics===
Let the set of [[Homogeneous polynomial|homogeneous]] [[Harmonic function|harmonic]] [[polynomial]]s of degree {{mvar|k}} on {{math|'''ℝ'''<sup>''n''</sup>}} be denoted by {{math|'''A'''<sub>''k''</sub>}}. The set {{math|'''A'''<sub>''k''</sub>}} consists of the [[solid spherical harmonics]] of degree {{mvar|k}}. The solid spherical harmonics play a similar role in higher dimensions to the Hermite polynomials in dimension one. Specifically, if {{math|''f''&thinsp;(''x'') {{=}} ''e''<sup>−π{{abs|''x''}}<sup>2</sup></sup>''P''(''x'')}} for some {{math|''P''(''x'')}} in {{math|'''A'''<sub>''k''</sub>}}, then {{math|''f̂''&thinsp;(''ξ'') {{=}} ''i''{{isup|−''k''}} ''f''&thinsp;(''ξ'')}}. Let the set {{math|'''H'''<sub>''k''</sub>}} be the closure in {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} of linear combinations of functions of the form {{math|''f''&thinsp;({{abs|''x''}})''P''(''x'')}} where {{math|''P''(''x'')}} is in {{math|'''A'''<sub>''k''</sub>}}. The space {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} is then a direct sum of the spaces {{math|'''H'''<sub>''k''</sub>}} and the Fourier transform maps each space {{math|'''H'''<sub>''k''</sub>}} to itself and is possible to characterize the action of the Fourier transform on each space {{math|'''H'''<sub>''k''</sub>}}.<ref name="Stein-Weiss-1971" />

Let {{math|''f''&thinsp;(''x'') {{=}} ''f''<sub>0</sub>({{abs|''x''}})''P''(''x'')}} (with {{math|''P''(''x'')}} in {{math|'''A'''<sub>''k''</sub>}}), then

:<math>\hat{f}(\xi)=F_0(|\xi|)P(\xi)</math>

where

:<math>F_0(r)=2\pi i^{-k}r^{-\frac{n+2k-2}{2}}\int_0^\infty f_0(s)J_\frac{n+2k-2}{2}(2\pi rs)s^\frac{n+2k}{2}\,ds.</math>

Here {{math|''J''<sub>{{sfrac|''n'' + 2''k'' − 2|2}}</sub>}} denotes the [[Bessel function]] of the first kind with order {{math|{{sfrac|''n'' + 2''k'' − 2|2}}}}. When {{math|''k'' {{=}} 0}} this gives a useful formula for the Fourier transform of a radial function.<ref>{{harvnb|Grafakos|2004}}.</ref> This is essentially the [[Hankel transform]]. Moreover, there is a simple recursion relating the cases {{math|''n'' + 2}} and {{mvar|n}}<ref>{{harvnb|Grafakos|Teschl|2013}}.</ref> allowing to compute, e.g., the three-dimensional Fourier transform of a radial function from the one-dimensional one.

===Restriction problems===
In higher dimensions it becomes interesting to study ''restriction problems'' for the Fourier transform. The Fourier transform of an integrable function is continuous and the restriction of this function to any set is defined. But for a square-integrable function the Fourier transform could be a general ''class'' of square integrable functions. As such, the restriction of the Fourier transform of an {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} function cannot be defined on sets of measure 0. It is still an active area of study to understand restriction problems in {{math|''L''{{isup|''p''}}}} for {{math|1 < ''p'' < 2}}. Surprisingly, it is possible in some cases to define the restriction of a Fourier transform to a set {{mvar|S}}, provided {{mvar|S}} has non-zero curvature. The case when {{mvar|S}} is the unit sphere in {{math|'''ℝ'''<sup>''n''</sup>}} is of particular interest. In this case the Tomas–[[Elias Stein|Stein]] restriction theorem states that the restriction of the Fourier transform to the unit sphere in {{math|'''ℝ'''<sup>''n''</sup>}} is a bounded operator on {{math|''L''{{isup|''p''}}}} provided {{math|1 ≤ ''p'' ≤ {{sfrac|2''n'' + 2|''n'' + 3}}}}.

One notable difference between the Fourier transform in 1 dimension versus higher dimensions concerns the partial sum operator. Consider an increasing collection of measurable sets {{math|''E''<sub>''R''</sub>}} indexed by {{math|''R'' ∈ (0,∞)}}: such as balls of radius {{mvar|R}} centered at the origin, or cubes of side {{math|2''R''}}. For a given integrable function {{mvar|f}}, consider the function {{mvar|f<sub>R</sub>}} defined by:

:<math>f_R(x) = \int_{E_R}\hat{f}(\xi) e^{2\pi ix\cdot\xi}\, d\xi, \quad x \in \mathbb{R}^n.</math>

Suppose in addition that {{math|''f'' ∈ ''L''{{isup|''p''}}('''ℝ'''<sup>''n''</sup>)}}. For {{math|''n'' {{=}} 1}} and {{math|1 < ''p'' < ∞}}, if one takes {{math|''E<sub>R</sub>'' {{=}} (−''R'', ''R'')}}, then {{mvar|f<sub>R</sub>}} converges to {{mvar|f}} in {{math|''L''{{isup|''p''}}}} as {{mvar|R}} tends to infinity, by the boundedness of the [[Hilbert transform]]. Naively one may hope the same holds true for {{math|''n'' > 1}}. In the case that {{mvar|E<sub>R</sub>}} is taken to be a cube with side length {{mvar|R}}, then convergence still holds. Another natural candidate is the Euclidean ball {{math|''E''<sub>''R''</sub> {{=}} {''ξ'' : {{abs|''ξ''}} < ''R''}|}}. In order for this partial sum operator to converge, it is necessary that the multiplier for the unit ball be bounded in {{math|''L''{{isup|''p''}}('''ℝ'''<sup>''n''</sup>)}}. For {{math|''n'' ≥ 2}} it is a celebrated theorem of [[Charles Fefferman]] that the multiplier for the unit ball is never bounded unless {{math|''p'' {{=}} 2}}.<ref name="Duoandikoetxea-2001" /> In fact, when {{math|''p'' ≠ 2}}, this shows that not only may {{mvar|f<sub>R</sub>}} fail to converge to {{mvar|f}} in {{math|''L''{{isup|''p''}}}}, but for some functions {{math|''f'' ∈ ''L''{{isup|''p''}}('''ℝ'''<sup>''n''</sup>)}}, {{mvar|f<sub>R</sub>}} is not even an element of {{math|''L''{{isup|''p''}}}}.

==Fourier transform on function spaces==

===On {{math|''L''{{isup|''p''}}}} spaces===

====On {{math|''L''<sup>1</sup>}}====

The definition of the Fourier transform by the integral formula

:<math>\hat{f}(\xi) = \int_{\mathbb{R}^n} f(x)e^{-2\pi i \xi\cdot x}\,dx</math>

is valid for Lebesgue integrable functions {{mvar|f}}; that is, {{math|''f'' ∈ ''L''<sup>1</sup>('''ℝ'''<sup>''n''</sup>)}}.

The Fourier transform {{math|{{mathcal|F}} : ''L''<sup>1</sup>('''ℝ'''<sup>''n''</sup>) → ''L''<sup>∞</sup>('''ℝ'''<sup>''n''</sup>)}} is a [[bounded operator]]. This follows from the observation that

:<math>\left\vert\hat{f}(\xi)\right\vert \leq \int_{\mathbb{R}^n} \vert f(x)\vert \,dx,</math>

which shows that its [[operator norm]] is bounded by 1. Indeed, it equals 1, which can be seen, for example, from the [[#rect|transform of the rect function]]. The image of {{math|''L''<sup>1</sup>}} is a subset of the space {{math|''C''<sub>0</sub>('''ℝ'''<sup>''n''</sup>)}} of continuous functions that tend to zero at infinity (the [[Riemann–Lebesgue lemma]]), although it is not the entire space. Indeed, there is no simple characterization of the image.

====On {{math|''L''<sup>2</sup>}}====

Since compactly supported smooth functions are integrable and dense in {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}}, the [[Plancherel theorem]] allows us to extend the definition of the Fourier transform to general functions in {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} by continuity arguments. The Fourier transform in {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} is no longer given by an ordinary Lebesgue integral, although it can be computed by an [[improper integral]], here meaning that for an {{math|''L''<sup>2</sup>}} function {{mvar|f}},

:<math>\hat{f}(\xi) = \lim_{R\to\infty}\int_{|x|\le R} f(x) e^{-2\pi i x\cdot\xi}\,dx</math>

where the limit is taken in the {{math|''L''<sup>2</sup>}} sense. (More generally, you can take a sequence of functions that are in the intersection of {{math|''L''<sup>1</sup>}} and {{math|''L''<sup>2</sup>}} and that converges to {{mvar|f}} in the {{math|''L''<sup>2</sup>}}-norm, and define the Fourier transform of {{mvar|f}} as the {{math|''L''<sup>2</sup>}} -limit of the Fourier transforms of these functions.<ref>{{cite web|url=https://statweb.stanford.edu/~candes/math262/Lectures/Lecture03.pdf|title=Applied Fourier Analysis and Elements of Modern Signal Processing Lecture 3 |date= January 12, 2016|access-date=2019-10-11}}</ref>)

Many of the properties of the Fourier transform in {{math|''L''<sup>1</sup>}} carry over to {{math|''L''<sup>2</sup>}}, by a suitable limiting argument.

Furthermore, {{math|{{mathcal|F}} : ''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>) → ''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} is a [[unitary operator]].<ref>{{harvnb|Stein|Weiss|1971|loc=Thm. 2.3}}.</ref> For an operator to be unitary it is sufficient to show that it is bijective and preserves the inner product, so in this case these follow from the Fourier inversion theorem combined with the fact that for any {{math|''f'', ''g'' ∈ ''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} we have

: <math>\int_{\mathbb{R}^n} f(x)\mathcal{F}g(x)\,dx = \int_{\mathbb{R}^n} \mathcal{F}f(x)g(x)\,dx. </math>

In particular, the image of {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}} is itself under the Fourier transform.

====On other {{math|''L''{{isup|''p''}}}}====

The definition of the Fourier transform can be extended to functions in {{math|''L''{{isup|''p''}}('''ℝ'''<sup>''n''</sup>)}} for {{math|1 ≤ ''p'' ≤ 2}} by decomposing such functions into a fat tail part in {{math|''L''<sup>2</sup>}} plus a fat body part in {{math|''L''<sup>1</sup>}}. In each of these spaces, the Fourier transform of a function in {{math|''L''{{isup|''p''}}('''ℝ'''<sup>''n''</sup>)}} is in {{math|''L''{{isup|''q''}}('''ℝ'''<sup>''n''</sup>)}}, where {{math|''q'' {{=}} {{sfrac|''p''|''p'' − 1}}}} is the Hölder conjugate of {{mvar|p}} (by the [[Hausdorff–Young inequality]]). However, except for {{math|''p'' {{=}} 2}}, the image is not easily characterized. Further extensions become more technical. The Fourier transform of functions in {{math|''L''{{isup|''p''}}}} for the range {{math|2 < ''p'' < ∞}} requires the study of distributions.<ref name="Katznelson-1976" /> In fact, it can be shown that there are functions in {{math|''L''{{isup|''p''}}}} with {{math|''p'' > 2}} so that the Fourier transform is not defined as a function.<ref name="Stein-Weiss-1971" />

===Tempered distributions===
{{Main|Distribution (mathematics)#Tempered distributions and Fourier transform}}
One might consider enlarging the domain of the Fourier transform from {{math|''L''<sup>1</sup> + ''L''<sup>2</sup>}} by considering [[generalized function]]s, or distributions. A distribution on {{math|'''ℝ'''<sup>''n''</sup>}} is a continuous linear functional on the space {{math|''C''<sub>c</sub>('''ℝ'''<sup>''n''</sup>)}} of compactly supported smooth functions, equipped with a suitable topology. The strategy is then to consider the action of the Fourier transform on {{math|''C''<sub>c</sub>('''ℝ'''<sup>''n''</sup>)}} and pass to distributions by duality. The obstruction to doing this is that the Fourier transform does not map {{math|''C''<sub>c</sub>('''ℝ'''<sup>''n''</sup>)}} to {{math|''C''<sub>c</sub>('''ℝ'''<sup>''n''</sup>)}}. In fact the Fourier transform of an element in {{math|''C''<sub>c</sub>('''ℝ'''<sup>''n''</sup>)}} can not vanish on an open set; see the above discussion on the uncertainty principle. The right space here is the slightly larger space of [[Schwartz space|Schwartz functions]]. The Fourier transform is an automorphism on the Schwartz space, as a topological vector space, and thus induces an automorphism on its dual, the space of tempered distributions.<ref name="Stein-Weiss-1971" /> The tempered distributions include all the integrable functions mentioned above, as well as well-behaved functions of polynomial growth and distributions of compact support.

For the definition of the Fourier transform of a tempered distribution, let {{mvar|f}} and {{mvar|g}} be integrable functions, and let {{math|''f̂''}} and {{math|''ĝ''}} be their Fourier transforms respectively. Then the Fourier transform obeys the following multiplication formula,<ref name="Stein-Weiss-1971" />

:<math>\int_{\mathbb{R}^n}\hat{f}(x)g(x)\,dx=\int_{\mathbb{R}^n}f(x)\hat{g}(x)\,dx.</math>

Every integrable function {{mvar|f}} defines (induces) a distribution {{mvar|T<sub>f</sub>}} by the relation

:<math>T_f(\varphi)=\int_{\mathbb{R}^n}f(x)\varphi(x)\,dx</math>

for all Schwartz functions {{mvar|φ}}. So it makes sense to define Fourier transform {{math|''T̂<sub>f</sub>''}} of {{math|''T<sub>f</sub>''}} by

:<math>\hat{T}_f (\varphi)= T_f\left(\hat{\varphi}\right)</math>

for all Schwartz functions {{mvar|φ}}. Extending this to all tempered distributions {{mvar|T}} gives the general definition of the Fourier transform.

Distributions can be differentiated and the above-mentioned compatibility of the Fourier transform with differentiation and convolution remains true for tempered distributions.

==Generalizations==

===Fourier–Stieltjes transform===
The Fourier transform of a finite Borel measure {{mvar|μ}} on {{math|'''ℝ'''<sup>''n''</sup>}} is given by:<ref>{{harvnb|Pinsky|2002|p=256}}.</ref>

:<math>\hat\mu(\xi)=\int_{\mathbb{R}^n} e^{-2\pi i x \cdot \xi}\,d\mu.</math>

This transform continues to enjoy many of the properties of the Fourier transform of integrable functions. One notable difference is that the [[Riemann–Lebesgue lemma]] fails for measures.<ref name="Katznelson-1976" /> In the case that {{math|''dμ'' {{=}} ''f''&thinsp;(''x'') ''dx''}}, then the formula above reduces to the usual definition for the Fourier transform of {{mvar|f}}. In the case that {{mvar|μ}} is the probability distribution associated to a random variable {{mvar|X}}, the Fourier–Stieltjes transform is closely related to the [[Characteristic function (probability theory)|characteristic function]], but the typical conventions in probability theory take {{math|''e''<sup>''ixξ''</sup>}} instead of {{math|''e''<sup>−2π''ixξ''</sup>}}.<ref name="Pinsky-2002" /> In the case when the distribution has a [[probability density function]] this definition reduces to the Fourier transform applied to the probability density function, again with a different choice of constants.

The Fourier transform may be used to give a characterization of measures. [[Bochner's theorem]] characterizes which functions may arise as the Fourier–Stieltjes transform of a positive measure on the circle.<ref name="Katznelson-1976" />

Furthermore, the [[Dirac delta function]], although not a function, is a finite [[Borel measure]]. Its Fourier transform is a constant function (whose specific value depends upon the form of the Fourier transform used).

===Locally compact abelian groups===
{{Main|Pontryagin duality}}
The Fourier transform may be generalized to any locally compact abelian group. A locally compact abelian group is an [[abelian group]] that is at the same time a [[locally compact]] [[Hausdorff space|Hausdorff topological space]] so that the group operation is continuous. If {{mvar|G}} is a locally compact abelian group, it has a translation invariant measure {{mvar|μ}}, called [[Haar measure]]. For a locally compact abelian group {{mvar|G}}, the set of irreducible, i.e. one-dimensional, unitary representations are called its [[character group|characters]]. With its natural group structure and the topology of pointwise convergence, the set of characters {{mvar|Ĝ}} is itself a locally compact abelian group, called the ''Pontryagin dual'' of {{mvar|G}}. For a function {{mvar|f}} in {{math|''L''<sup>1</sup>(''G'')}}, its Fourier transform is defined by<ref name="Katznelson-1976" />

:<math>\hat{f}(\xi)=\int_G \xi(x)f(x)\,d\mu\qquad\text{for any }\xi\in\hat G.</math>

The Riemann–Lebesgue lemma holds in this case; {{math|''f̂''&thinsp;(''ξ'')}} is a function vanishing at infinity on {{mvar|Ĝ}}.

===Gelfand transform===
{{Main|Gelfand representation}}
The Fourier transform is also a special case of [[Gelfand transform]]. In this particular context, it is closely related to the Pontryagin duality map defined above.

Given an abelian [[locally compact space|locally compact]] [[Hausdorff space|Hausdorff]] [[topological group]] {{mvar|G}}, as before we consider space {{math|''L''<sup>1</sup>(''G'')}}, defined using a Haar measure. With convolution as multiplication, {{math|''L''<sup>1</sup>(''G'')}} is an abelian [[Banach algebra]]. It also has an [[Involution (mathematics)|involution]] * given by

:<math>f^*(g) = \overline{f\left(g^{-1}\right)}.</math>

Taking the completion with respect to the largest possibly {{math|''C''*}}-norm gives its enveloping {{math|''C''*}}-algebra, called the group {{math|''C''*}}-algebra {{math|''C''*(''G'')}} of {{mvar|G}}. (Any {{math|''C''*}}-norm on {{math|''L''<sup>1</sup>(''G'')}} is bounded by the {{math|''L''<sup>1</sup>}} norm, therefore their supremum exists.)

Given any abelian {{math|''C''*}}-algebra {{mvar|A}}, the Gelfand transform gives an isomorphism between {{mvar|A}} and {{math|''C''<sub>0</sub>(''A''^)}}, where {{math|''A''^}} is the multiplicative linear functionals, i.e. one-dimensional representations, on {{mvar|A}} with the weak-* topology. The map is simply given by

:<math>a \mapsto \bigl( \varphi \mapsto \varphi(a) \bigr)</math>

It turns out that the multiplicative linear functionals of {{math|''C''*(''G'')}}, after suitable identification, are exactly the characters of {{mvar|G}}, and the Gelfand transform, when restricted to the dense subset {{math|''L''<sup>1</sup>(''G'')}} is the Fourier–Pontryagin transform.

===Compact non-abelian groups===
The Fourier transform can also be defined for functions on a non-abelian group, provided that the group is [[compact space|compact]]. Removing the assumption that the underlying group is abelian, irreducible unitary representations need not always be one-dimensional. This means the Fourier transform on a non-abelian group takes values as Hilbert space operators.<ref>{{harvnb|Hewitt|Ross|1970|loc=Chapter 8}}.</ref> The Fourier transform on compact groups is a major tool in [[representation theory]]<ref>{{harvnb|Knapp|2001}}.</ref> and [[non-commutative harmonic analysis]].

Let {{mvar|G}} be a compact [[Hausdorff space|Hausdorff]] [[topological group]]. Let {{math|Σ}} denote the collection of all isomorphism classes of finite-dimensional irreducible [[unitary representation]]s, along with a definite choice of representation {{math|''U''{{isup|(''σ'')}}}} on the [[Hilbert space]] {{math|''H<sub>σ</sub>''}} of finite dimension {{math|''d<sub>σ</sub>''}} for each {{math|''σ'' ∈ Σ}}. If {{mvar|μ}} is a finite [[Borel measure]] on {{mvar|G}}, then the Fourier–Stieltjes transform of {{mvar|μ}} is the operator on {{math|''H<sub>σ</sub>''}} defined by

:<math>\left\langle \hat{\mu}\xi,\eta\right\rangle_{H_\sigma} = \int_G \left\langle \overline{U}^{(\sigma)}_g\xi,\eta\right\rangle\,d\mu(g)</math>

where {{math|{{overline|''U''}}{{isup|(''σ'')}}}} is the complex-conjugate representation of {{math|''U''<sup>(''σ'')</sup>}} acting on {{math|''H<sub>σ</sub>''}}. If {{mvar|μ}} is [[absolutely continuous]] with respect to the [[Haar measure|left-invariant probability measure]] {{mvar|λ}} on {{mvar|G}}, [[Radon–Nikodym theorem|represented]] as

:<math>d\mu = f \, d\lambda</math>

for some {{math|''f'' ∈ [[Lp space|''L''<sup>1</sup>(''λ'')]]}}, one identifies the Fourier transform of {{mvar|f}} with the Fourier–Stieltjes transform of {{mvar|μ}}.

The mapping
:<math>\mu\mapsto\hat{\mu}</math>
defines an isomorphism between the [[Banach space]] {{math|''M''(''G'')}} of finite Borel measures (see [[rca space]]) and a closed subspace of the Banach space {{math|'''C'''<sub>∞</sub>(Σ)}} consisting of all sequences {{math|''E'' {{=}} (''E<sub>σ</sub>'')}} indexed by {{math|Σ}} of (bounded) linear operators {{math|''E<sub>σ</sub>'' : ''H<sub>σ</sub>'' → ''H<sub>σ</sub>''}} for which the norm

:<math>\|E\| = \sup_{\sigma\in\Sigma}\left\|E_\sigma\right\|</math>

is finite. The "[[convolution theorem]]" asserts that, furthermore, this isomorphism of Banach spaces is in fact an isometric isomorphism of [[C* algebra]]s into a subspace of {{math|'''C'''<sub>∞</sub>(Σ)}}. Multiplication on {{math|''M''(''G'')}} is given by [[convolution]] of measures and the involution * defined by

:<math>f^*(g) = \overline{f\left(g^{-1}\right)},</math>

and {{math|'''C'''<sub>∞</sub>(Σ)}} has a natural {{math|''C''*}}-algebra structure as Hilbert space operators.

The [[Peter–Weyl theorem]] holds, and a version of the Fourier inversion formula ([[Plancherel's theorem]]) follows: if {{math|''f'' ∈ ''L''<sup>2</sup>(''G'')}}, then

:<math>f(g) = \sum_{\sigma\in\Sigma} d_\sigma \operatorname{tr}\left(\hat{f}(\sigma)U^{(\sigma)}_g\right)</math>

where the summation is understood as convergent in the {{math|''L''<sup>2</sup>}} sense.

The generalization of the Fourier transform to the noncommutative situation has also in part contributed to the development of [[noncommutative geometry]].{{Citation needed|date=May 2009}} In this context, a categorical generalization of the Fourier transform to noncommutative groups is [[Tannaka–Krein duality]], which replaces the group of characters with the category of representations. However, this loses the connection with harmonic functions.

==Alternatives==
In [[signal processing]] terms, a function (of time) is a representation of a signal with perfect ''time resolution'', but no frequency information, while the Fourier transform has perfect ''frequency resolution'', but no time information: the magnitude of the Fourier transform at a point is how much frequency content there is, but location is only given by phase (argument of the Fourier transform at a point), and [[Standing wave|standing waves]] are not localized in time – a sine wave continues out to infinity, without decaying. This limits the usefulness of the Fourier transform for analyzing signals that are localized in time, notably [[transient (acoustics)|transients]], or any signal of finite extent.

As alternatives to the Fourier transform, in [[time-frequency analysis]], one uses time-frequency transforms or time-frequency distributions to represent signals in a form that has some time information and some frequency information – by the uncertainty principle, there is a trade-off between these. These can be generalizations of the Fourier transform, such as the [[short-time Fourier transform]] or [[fractional Fourier transform]], or other functions to represent signals, as in [[Wavelet transform|wavelet transforms]] and [[Chirplet transform|chirplet transforms]], with the wavelet analog of the (continuous) Fourier transform being the [[continuous wavelet transform]].<ref name="Boashash-2003" />

==Applications==
{{see also|Spectral density#Applications}}

[[File:Commutative diagram illustrating problem solving via the Fourier transform.svg|thumb|400px|Some problems, such as certain differential equations, become easier to solve when the Fourier transform is applied. In that case the solution to the original problem is recovered using the inverse Fourier transform.]]

===Analysis of differential equations===
Perhaps the most important use of the Fourier transformation is to solve [[partial differential equation]]s.
Many of the equations of the mathematical physics of the nineteenth century can be treated this way. Fourier studied the heat equation, which in one dimension and in dimensionless units is

:<math>\frac{\partial^2 y(x,t)}{ \partial^2 x } =\frac{\partial y(x,t) }{ \partial t}.</math>

The example we will give, a slightly more difficult one, is the wave equation in one dimension,

:<math>\frac{\partial^2y(x,t)}{\partial^2 x }=\frac{\partial^2y(x,t)}{\partial^2t}.</math>

As usual, the problem is not to find a solution: there are infinitely many. The problem is that of the so-called "boundary problem": find a solution which satisfies the "boundary conditions"

:<math>y(x,0) = f(x) , \qquad \frac{\partial y(x,0) }{ \partial t}= g(x).</math>

Here, {{mvar|f}} and {{mvar|g}} are given functions. For the heat equation, only one boundary condition can be required (usually the first one). But for the wave equation, there are still infinitely many solutions {{mvar|y}} which satisfy the first boundary condition. But when one imposes both conditions, there is only one possible solution.

It is easier to find the Fourier transform {{mvar|ŷ}} of the solution than to find the solution directly. This is because the Fourier transformation takes differentiation into multiplication by the Fourier-dual variable, and so a partial differential equation applied to the original function is transformed into multiplication by polynomial functions of the dual variables applied to the transformed function. After {{mvar|ŷ}} is determined, we can apply the inverse Fourier transformation to find {{mvar|y}}.

Fourier's method is as follows. First, note that any function of the forms

:<math> \cos\bigl(2\pi\xi(x\pm t)\bigr) \mbox{ or } \sin\bigl(2\pi\xi(x \pm t)\bigr)</math>

satisfies the wave equation. These are called the elementary solutions.

Second, note that therefore any integral

:<math>y(x,t) = \int_{0}^{\infty} a_+(\xi)\cos\bigl(2\pi\xi(x +t)\bigr) + a_-(\xi)\cos\bigl(2\pi\xi(x -t)\bigr) + b_+(\xi)\sin\bigl(2\pi\xi(x +t)\bigr) + b_-(\xi)\sin\left(2\pi\xi(x -t)\right) \, d\xi</math>

(for arbitrary {{math|''a''<sub>+</sub>, ''a''<sub>−</sub>, ''b''<sub>+</sub>, ''b''<sub>−</sub>}}) satisfies the wave equation. (This integral is just a kind of continuous linear combination, and the equation is linear.)

Now this resembles the formula for the Fourier synthesis of a function. In fact, this is the real inverse Fourier transform of {{math|''a''<sub>±</sub>}} and {{math|''b''<sub>±</sub>}} in the variable {{mvar|x}}.

The third step is to examine how to find the specific unknown coefficient functions {{math|''a''<sub>±</sub>}} and {{math|''b''<sub>±</sub>}} that will lead to {{mvar|y}} satisfying the boundary conditions. We are interested in the values of these solutions at {{math|''t'' {{=}} 0}}. So we will set {{math|''t'' {{=}} 0}}. Assuming that the conditions needed for Fourier inversion are satisfied, we can then find the Fourier sine and cosine transforms (in the variable {{mvar|x}}) of both sides and obtain

:<math> 2\int_{-\infty}^\infty y(x,0) \cos(2\pi\xi x) \, dx = a_++a_-</math>

and

:<math>2\int_{-\infty}^\infty y(x,0) \sin(2\pi\xi x) \, dx = b_++b_-.</math>

Similarly, taking the derivative of {{mvar|y}} with respect to {{mvar|t}} and then applying the Fourier sine and cosine transformations yields

:<math> 2\int_{-\infty}^{\infty} \frac{\partial y(u,0)}{\partial t} \sin (2\pi\xi x) \, dx = (2\pi\xi)\left(-a_++a_-\right)</math>

and

:<math>2\int_{-\infty}^\infty \frac{\partial y(u,0)}{\partial t} \cos (2\pi\xi x) \, dx = (2\pi\xi)\left(b_+-b_-\right).</math>

These are four linear equations for the four unknowns {{math|''a''<sub>±</sub>}} and {{math|''b''<sub>±</sub>}}, in terms of the Fourier sine and cosine transforms of the boundary conditions, which are easily solved by elementary algebra, provided that these transforms can be found.

In summary, we chose a set of elementary solutions, parametrised by {{mvar|ξ}}, of which the general solution would be a (continuous) linear combination in the form of an integral over the parameter {{mvar|ξ}}. But this integral was in the form of a Fourier integral. The next step was to express the boundary conditions in terms of these integrals, and set them equal to the given functions {{mvar|f}} and {{mvar|g}}. But these expressions also took the form of a Fourier integral because of the properties of the Fourier transform of a derivative. The last step was to exploit Fourier inversion by applying the Fourier transformation to both sides, thus obtaining expressions for the coefficient functions {{math|''a''<sub>±</sub>}} and {{math|''b''<sub>±</sub>}} in terms of the given boundary conditions {{mvar|f}} and {{mvar|g}}.

From a higher point of view, Fourier's procedure can be reformulated more conceptually. Since there are two variables, we will use the Fourier transformation in both {{mvar|x}} and {{mvar|t}} rather than operate as Fourier did, who only transformed in the spatial variables. Note that {{mvar|ŷ}} must be considered in the sense of a distribution since {{math|''y''(''x'', ''t'')}} is not going to be {{math|''L''<sup>1</sup>}}: as a wave, it will persist through time and thus is not a transient phenomenon. But it will be bounded and so its Fourier transform can be defined as a distribution. The operational properties of the Fourier transformation that are relevant to this equation are that it takes differentiation in {{mvar|x}} to multiplication by {{math|2π''iξ''}} and differentiation with respect to {{mvar|t}} to multiplication by {{math|2π''if''}} where {{mvar|f}} is the frequency. Then the wave equation becomes an algebraic equation in {{mvar|ŷ}}:

:<math>\xi^2 \hat y (\xi, f) = f^2 \hat y (\xi, f).</math>

This is equivalent to requiring {{math|''ŷ''(''ξ'', ''f''&thinsp;) {{=}} 0}} unless {{math|''ξ'' {{=}} ±''f''}}. Right away, this explains why the choice of elementary solutions we made earlier worked so well: obviously {{math|''f̂'' {{=}} ''δ''(''ξ'' ± ''f''&thinsp;)}} will be solutions. Applying Fourier inversion to these delta functions, we obtain the elementary solutions we picked earlier. But from the higher point of view, one does not pick elementary solutions, but rather considers the space of all distributions which are supported on the (degenerate) conic {{math|''ξ''{{isup|2}} − ''f''{{isup|2}} {{=}} 0}}.

We may as well consider the distributions supported on the conic that are given by distributions of one variable on the line {{math|''ξ'' {{=}} ''f''}} plus distributions on the line {{math|''ξ'' {{=}} −''f''}} as follows: if {{mvar|ϕ}} is any test function,

:<math>\iint \hat y \phi(\xi,f) \, d\xi \, df = \int s_+ \phi(\xi,\xi) \, d\xi + \int s_- \phi(\xi,-\xi) \, d\xi,</math>

where {{math|''s''<sub>+</sub>}}, and {{math|''s''<sub>−</sub>}}, are distributions of one variable.

Then Fourier inversion gives, for the boundary conditions, something very similar to what we had more concretely above (put {{math|''ϕ''(''ξ'', ''f''&thinsp;) {{=}} ''e''<sup>2π''i''(''xξ''+''tf''&thinsp;)</sup>}}, which is clearly of polynomial growth):

:<math> y(x,0) = \int\bigl\{s_+(\xi) + s_-(\xi)\bigr\} e^{2\pi i\xi x+0} \, d\xi </math>

and

:<math> \frac{\partial y(x,0)}{\partial t} = \int\bigl\{s_+(\xi) - s_-(\xi)\bigr\} 2\pi i \xi e^{2\pi i\xi x+0} \, d\xi.</math>

Now, as before, applying the one-variable Fourier transformation in the variable {{mvar|x}} to these functions of {{mvar|x}} yields two equations in the two unknown distributions {{math|''s''<sub>±</sub>}} (which can be taken to be ordinary functions if the boundary conditions are {{math|''L''<sup>1</sup>}} or {{math|''L''<sup>2</sup>}}).

From a calculational point of view, the drawback of course is that one must first calculate the Fourier transforms of the boundary conditions, then assemble the solution from these, and then calculate an inverse Fourier transform. Closed form formulas are rare, except when there is some geometric symmetry that can be exploited, and the numerical calculations are difficult because of the oscillatory nature of the integrals, which makes convergence slow and hard to estimate. For practical calculations, other methods are often used.

The twentieth century has seen the extension of these methods to all linear partial differential equations with polynomial coefficients, and by extending the notion of Fourier transformation to include Fourier integral operators, some non-linear equations as well.

===Fourier transform spectroscopy===
{{main|Fourier transform spectroscopy}}
The Fourier transform is also used in [[nuclear magnetic resonance]] (NMR) and in other kinds of [[spectroscopy]], e.g. infrared ([[Fourier-transform infrared spectroscopy|FTIR]]). In NMR an exponentially shaped free induction decay (FID) signal is acquired in the time domain and Fourier-transformed to a Lorentzian line-shape in the frequency domain. The Fourier transform is also used in [[magnetic resonance imaging]] (MRI) and [[mass spectrometry]].

===Quantum mechanics===

The Fourier transform is useful in [[quantum mechanics]] in two different ways. To begin with, the basic conceptual structure of quantum mechanics postulates the existence of pairs of [[complementary variables]], connected by the [[Heisenberg uncertainty principle]]. For example, in one dimension, the spatial variable {{mvar|q}} of, say, a particle, can only be measured by the quantum mechanical "[[position operator]]" at the cost of losing information about the momentum {{mvar|p}} of the particle. Therefore, the physical state of the particle can either be described by a function, called "the wave function", of {{mvar|q}} or by a function of {{mvar|p}} but not by a function of both variables. The variable {{mvar|p}} is called the conjugate variable to {{mvar|q}}. In classical mechanics, the physical state of a particle (existing in one dimension, for simplicity of exposition) would be given by assigning definite values to both {{mvar|p}} and {{mvar|q}} simultaneously. Thus, the set of all possible physical states is the two-dimensional real vector space with a {{mvar|p}}-axis and a {{mvar|q}}-axis called the [[phase space]].

In contrast, quantum mechanics chooses a polarisation of this space in the sense that it picks a subspace of one-half the dimension, for example, the {{mvar|q}}-axis alone, but instead of considering only points, takes the set of all complex-valued "wave functions" on this axis. Nevertheless, choosing the {{mvar|p}}-axis is an equally valid polarisation, yielding a different representation of the set of possible physical states of the particle which is related to the first representation by the Fourier transformation

:<math>\phi(p) = \int \psi (q) e^{2\pi i \frac{pq}{h}}\, dq.</math>

Physically realisable states are {{math|''L''<sup>2</sup>}}, and so by the Plancherel theorem, their Fourier transforms are also {{math|''L''<sup>2</sup>}}.  (Note that since {{mvar|q}} is in units of distance and {{mvar|p}} is in units of momentum, the presence of Planck's constant in the exponent makes the exponent [[Nondimensionalization|dimensionless]], as it should be.)

Therefore, the Fourier transform can be used to pass from one way of representing the state of the particle, by a wave function of position, to another way of representing the state of the particle: by a wave function of momentum. Infinitely many different polarisations are possible, and all are equally valid. Being able to transform states from one representation to another is sometimes convenient.

The other use of the Fourier transform in both quantum mechanics and [[quantum field theory]] is to solve the applicable wave equation. In non-relativistic quantum mechanics, [[Schrödinger's equation]] for a time-varying wave function in one-dimension, not subject to external forces, is

:<math>\frac{\partial^2}{\partial x^2} \psi(x,t) = i \frac h{2\pi} \frac{\partial}{\partial t} \psi(x,t).</math>

This is the same as the heat equation except for the presence of the imaginary unit {{mvar|i}}. Fourier methods can be used to solve this equation.

In the presence of a potential, given by the potential energy function {{math|''V''(''x'')}}, the equation becomes

:<math>\frac{\partial^2}{\partial x^2} \psi(x,t) + V(x)\psi(x,t) = i \frac h{2\pi} \frac{\partial}{\partial t} \psi(x,t).</math>

The "elementary solutions", as we referred to them above, are the so-called "stationary states" of the particle, and Fourier's algorithm, as described above, can still be used to solve the boundary value problem of the future evolution of {{mvar|ψ}} given its values for {{math|''t'' {{=}} 0}}. Neither of these approaches is of much practical use in quantum mechanics. Boundary value problems and the time-evolution of the wave function is not of much practical interest: it is the stationary states that are most important.

In relativistic quantum mechanics, Schrödinger's equation becomes a wave equation as was usual in classical physics, except that complex-valued waves are considered. A simple example, in the absence of interactions with other particles or fields, is the free one-dimensional Klein–Gordon–Schrödinger–Fock equation, this time in dimensionless units,

:<math>\left (\frac{\partial^2}{\partial x^2} +1 \right) \psi(x,t) = \frac{\partial^2}{\partial t^2} \psi(x,t).</math>

This is, from the mathematical point of view, the same as the wave equation of classical physics solved above (but with a complex-valued wave, which makes no difference in the methods). This is of great use in quantum field theory: each separate Fourier component of a wave can be treated as a separate harmonic oscillator and then quantized, a procedure known as "second quantization". Fourier methods have been adapted to also deal with non-trivial interactions.

===Signal processing===
The Fourier transform is used for the spectral analysis of time-series. The subject of statistical signal processing does not, however, usually apply the Fourier transformation to the signal itself. Even if a real signal is indeed transient, it has been found in practice advisable to model a signal by a function (or, alternatively, a stochastic process) which is stationary in the sense that its characteristic properties are constant over all time. The Fourier transform of such a function does not exist in the usual sense, and it has been found more useful for the analysis of signals to instead take the Fourier transform of its autocorrelation function.

The autocorrelation function {{mvar|R}} of a function {{mvar|f}} is defined by

:<math>R_f (\tau) = \lim_{T\rightarrow \infty} \frac{1}{2T} \int_{-T}^T f(t) f(t+\tau) \, dt. </math>

This function is a function of the time-lag {{mvar|τ}} elapsing between the values of {{mvar|f}} to be correlated.

For most functions {{mvar|f}} that occur in practice, {{mvar|R}} is a bounded even function of the time-lag {{mvar|τ}} and for typical noisy signals it turns out to be uniformly continuous with a maximum at {{math|''τ'' {{=}} 0}}.

The autocorrelation function, more properly called the autocovariance function unless it is normalized in some appropriate fashion, measures the strength of the correlation between the values of {{mvar|f}} separated by a time lag. This is a way of searching for the correlation of {{mvar|f}} with its own past. It is useful even for other statistical tasks besides the analysis of signals. For example, if {{math|''f''&thinsp;(''t'')}} represents the temperature at time {{mvar|t}}, one expects a strong correlation with the temperature at a time lag of 24 hours.

It possesses a Fourier transform,

:<math> P_f(\xi) = \int_{-\infty}^\infty R_f (\tau) e^{-2\pi i \xi\tau} \, d\tau. </math>

This Fourier transform is called the power spectral density function of {{mvar|f}}. (Unless all periodic components are first filtered out from {{mvar|f}}, this integral will diverge, but it is easy to filter out such periodicities.)

The power spectrum, as indicated by this density function {{mvar|P}}, measures the amount of variance contributed to the data by the frequency {{mvar|ξ}}. In electrical signals, the variance is proportional to the average power (energy per unit time), and so the power spectrum describes how much the different frequencies contribute to the average power of the signal. This process is called the spectral analysis of time-series and is analogous to the usual analysis of variance of data that is not a time-series ([[ANOVA]]).

Knowledge of which frequencies are "important" in this sense is crucial for the proper design of filters and for the proper evaluation of measuring apparatuses. It can also be useful for the scientific analysis of the phenomena responsible for producing the data.

The power spectrum of a signal can also be approximately measured directly by measuring the average power that remains in a signal after all the frequencies outside a narrow band have been filtered out.

Spectral analysis is carried out for visual signals as well. The power spectrum ignores all phase relations, which is good enough for many purposes, but for video signals other types of spectral analysis must also be employed, still using the Fourier transform as a tool.

==Other notations==
Other common notations for {{math|''f̂''&thinsp;(''ξ'')}} include:

:<math>\tilde{f}(\xi),\ \tilde{f}(\omega),\ F(\xi),\ \mathcal{F}\left(f\right)(\xi),\ \left(\mathcal{F}f\right)(\xi),\ \mathcal{F}(f),\ \mathcal F(\omega),\ F(\omega),\ \mathcal F(j\omega),\ \mathcal{F}\{f\},\ \mathcal{F} \bigl(f(t)\bigr),\ \mathcal{F} \bigl\{f(t)\bigr\}.</math>

Denoting the Fourier transform by a capital letter corresponding to the letter of function being transformed (such as {{math|''f''&thinsp;(''x'')}} and {{math|''F''(''ξ'')}}) is especially common in the sciences and engineering. In electronics, omega ({{mvar|ω}}) is often used instead of {{mvar|ξ}} due to its interpretation as angular frequency, sometimes it is written as {{math|''F''(&thinsp;''jω'')}}, where {{mvar|j}} is the [[imaginary unit]], to indicate its relationship with the [[Laplace transform]], and sometimes it is written informally as {{math|''F''(2π''f''&thinsp;)}} in order to use ordinary frequency. In some contexts such as particle physics, the same symbol <math>f</math> may be used for both for a function as well as it Fourier transform, with the two only distinguished by their [[Argument of a function|argument]]: <math>f(k_1 + k_2)</math> would refer to the Fourier transform because of the momentum argument, while <math>f(x_0 + \pi \vec r)</math> would refer to the original function because of the positional argument. Although tildes may be used as in <math>\tilde{f}</math> to indicate Fourier transforms, tildes may also be used to indicate a modification of a quantity with a more [[Lorentz invariant]] form, such as <math>\tilde{dk} = \frac{dk}{(2\pi)^32\omega}</math>, so care must be taken.

The interpretation of the complex function {{math|''f̂''&thinsp;(''ξ'')}} may be aided by expressing it in [[polar coordinate]] form

:<math>\hat f(\xi) = A(\xi) e^{i\varphi(\xi)}</math>

in terms of the two real functions {{math|''A''(''ξ'')}} and {{math|''φ''(''ξ'')}} where:

:<math>A(\xi) = \left|\hat f(\xi)\right|,</math>

is the [[amplitude]] and

:<math>\varphi (\xi) = \arg \left( \hat f(\xi) \right), </math>

is the [[phase (waves)|phase]] (see [[Arg (mathematics)|arg function]]).

Then the inverse transform can be written:

:<math>f(x) = \int _{-\infty}^\infty A(\xi)\ e^{ i\bigl(2\pi \xi x +\varphi (\xi)\bigr)}\,d\xi,</math>

which is a recombination of all the frequency components of {{math|''f''&thinsp;(''x'')}}. Each component is a complex [[sinusoid]] of the form {{math|''e''<sup>2π''ixξ''</sup>}} whose amplitude is {{math|''A''(''ξ'')}} and whose initial [[phase angle]] (at {{math|''x'' {{=}} 0}}) is {{math|''φ''(''ξ'')}}.

The Fourier transform may be thought of as a mapping on function spaces. This mapping is here denoted {{mathcal|F}} and {{math|{{mathcal|F}}(&thinsp;''f''&thinsp;)}} is used to denote the Fourier transform of the function {{mvar|f}}. This mapping is linear, which means that {{mathcal|F}} can also be seen as a linear transformation on the function space and implies that the standard notation in linear algebra of applying a linear transformation to a vector (here the function {{math|''f''&thinsp;}}) can be used to write {{math|{{mathcal|F}} ''f''}} instead of {{math|{{mathcal|F}}(&thinsp;''f''&thinsp;)}}. Since the result of applying the Fourier transform is again a function, we can be interested in the value of this function evaluated at the value {{mvar|ξ}} for its variable, and this is denoted either as {{math|{{mathcal|F}} ''f''&thinsp;(''ξ'')}} or as {{math|(&thinsp;{{mathcal|F}} ''f''&thinsp;)(''ξ'')}}. Notice that in the former case, it is implicitly understood that {{mathcal|F}} is applied first to {{mvar|f}} and then the resulting function is evaluated at {{mvar|ξ}}, not the other way around.

In mathematics and various applied sciences, it is often necessary to distinguish between a function {{mvar|f}} and the value of {{mvar|f}} when its variable equals {{mvar|x}}, denoted {{math|''f''&thinsp;(''x'')}}. This means that a notation like {{math|{{mathcal|F}}(&thinsp;''f''&thinsp;(''x''))}} formally can be interpreted as the Fourier transform of the values of {{mvar|f}} at {{mvar|x}}. Despite this flaw, the previous notation appears frequently, often when a particular function or a function of a particular variable is to be transformed. For example,

:<math>\mathcal F\bigl( \operatorname{rect}(x) \bigr) = \operatorname{sinc}(\xi)</math>

is sometimes used to express that the Fourier transform of a [[rectangular function]] is a [[sinc function]], or

:<math>\mathcal F\bigl(f(x + x_0)\bigr) = \mathcal F\bigl(f(x)\bigr) e^{2\pi i \xi x_0}</math>

is used to express the shift property of the Fourier transform.

Notice, that the last example is only correct under the assumption that the transformed function is a function of {{mvar|x}}, not of {{math|''x''<sub>0</sub>}}.

==Other conventions==
The Fourier transform can also be written in terms of [[angular frequency]]:

:<math>\omega = 2\pi \xi,</math>

whose units are [[radian]]s per second.

The substitution {{math|''ξ'' {{=}} {{sfrac|''ω''|2π}}}} into the formulas above produces this convention:

:<math>\hat{f}(\omega) = \int_{\mathbb R^n} f(x) e^{-i\omega\cdot x}\, dx.</math>

Under this convention, the inverse transform becomes:

:<math>f(x) = \frac{1}{(2\pi)^n} \int_{\mathbb R^n} \hat{f}(\omega)e^{i\omega \cdot x}\, d\omega.</math>

Unlike the convention followed in this article, when the Fourier transform is defined this way, it is no longer a [[unitary transformation]] on {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}}. There is also less symmetry between the formulas for the Fourier transform and its inverse.

Another convention is to split the factor of {{math|(2π)<sup>''n''</sup>}} evenly between the Fourier transform and its inverse, which leads to definitions:

:<math>\begin{align}
\hat{f}(\omega) &= \frac{1}{(2\pi)^\frac{n}{2}} \int_{\mathbb{R}^n} f(x) e^{- i\omega\cdot x}\, dx, \\
f(x) &= \frac{1}{(2\pi)^\frac{n}{2}} \int_{\mathbb{R}^n} \hat{f}(\omega) e^{ i\omega \cdot x}\, d\omega.
\end{align}</math>

Under this convention, the Fourier transform is again a unitary transformation on {{math|''L''<sup>2</sup>('''ℝ'''<sup>''n''</sup>)}}. It also restores the symmetry between the Fourier transform and its inverse.

Variations of all three conventions can be created by conjugating the complex-exponential [[integral kernel|kernel]] of both the forward and the reverse transform. The signs must be opposites. Other than that, the choice is (again) a matter of convention.

{|class="wikitable"
|+ Summary of popular forms of the Fourier transform, one-dimensional
|-
! ordinary frequency {{mvar|ξ}} (Hz)
! unitary
| <math>\begin{align}
\hat{f}_1(\xi)\ &\stackrel{\mathrm{def}}{=}\ \int_{-\infty}^{\infty} f(x)\cdot e^{-2 \pi i x\cdot\xi}\, dx = \sqrt{2 \pi}\cdot \hat{f}_2(2 \pi \xi) = \hat{f}_3(2 \pi \xi) \\
f(x) &= \int_{-\infty}^{\infty} \hat{f}_1(\xi)\cdot e^{2 \pi i x\cdot \xi}\, d\xi \end{align}</math>
|-
! rowspan=2 | angular frequency {{mvar|ω}} (rad/s)
! unitary
| <math>\begin{align}
\hat{f}_2(\omega)\ &\stackrel{\mathrm{def}}{=}\ \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^{\infty} f(x)\cdot  e^{-i \omega\cdot x}\, dx = \frac{1}{\sqrt{2 \pi}}\cdot \hat{f}_1 \! \left(\frac{\omega}{2 \pi}\right) = \frac{1}{\sqrt{2 \pi}}\cdot \hat{f}_3(\omega) \\
f(x) &= \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^{\infty} \hat{f}_2(\omega)\cdot e^{i \omega\cdot x}\, d\omega \end{align}</math>
|-
! non-unitary
| <math>\begin{align}
\hat{f}_3(\omega) \ &\stackrel{\mathrm{def}}{=}\ \int_{-\infty}^{\infty} f(x)\cdot  e^{-i\omega\cdot x}\, dx = \hat{f}_1 \left(\frac{\omega}{2 \pi}\right) = \sqrt{2 \pi}\cdot \hat{f}_2(\omega) \\
f(x) &= \frac{1}{2 \pi} \int_{-\infty}^{\infty} \hat{f}_3(\omega)\cdot e^{i \omega\cdot x}\, d\omega \end{align}</math>
|}

{|class="wikitable"
|+ Generalization for {{math|''n''}}-dimensional functions
|-
! ordinary frequency {{mvar|ξ}} (Hz)
! unitary
| <math>\begin{align}
\hat{f}_1(\xi)\ &\stackrel{\mathrm{def}}{=}\ \int_{\mathbb{R}^n} f(x) e^{-2 \pi i x\cdot\xi}\, dx = (2 \pi)^\frac{n}{2}\hat{f}_2(2 \pi \xi) = \hat{f}_3(2 \pi \xi) \\
f(x) &= \int_{\mathbb{R}^n} \hat{f}_1(\xi) e^{2 \pi i x\cdot \xi}\, d\xi \end{align}</math>
|-
! rowspan=2 | angular frequency {{mvar|ω}} (rad/s)
! unitary
| <math>\begin{align}
\hat{f}_2(\omega)\ &\stackrel{\mathrm{def}}{=}\ \frac{1}{(2 \pi)^\frac{n}{2}} \int_{\mathbb{R}^n} f(x) e^{-i \omega\cdot x}\, dx = \frac{1}{(2 \pi)^\frac{n}{2}} \hat{f}_1 \! \left(\frac{\omega}{2 \pi}\right) = \frac{1}{(2 \pi)^\frac{n}{2}} \hat{f}_3(\omega) \\
f(x) &= \frac{1}{(2 \pi)^\frac{n}{2}} \int_{\mathbb{R}^n} \hat{f}_2(\omega)e^{i \omega\cdot x}\, d\omega \end{align}</math>
|-
! non-unitary
| <math>\begin{align}
\hat{f}_3(\omega) \ &\stackrel{\mathrm{def}}{=}\ \int_{\mathbb{R}^n} f(x) e^{-i\omega\cdot x}\, dx = \hat{f}_1 \left(\frac{\omega}{2 \pi}\right) = (2 \pi)^\frac{n}{2} \hat{f}_2(\omega) \\
f(x) &= \frac{1}{(2 \pi)^n} \int_{\mathbb{R}^n} \hat{f}_3(\omega) e^{i \omega\cdot x}\, d\omega \end{align}</math>
|}

As discussed above, the [[Characteristic function (probability theory)|characteristic function]] of a random variable is the same as the [[Fourier transform#Fourier–Stieltjes transform|Fourier–Stieltjes transform]] of its distribution measure, but in this context it is typical to take a different convention for the constants. Typically characteristic function is defined
:<math>E\left(e^{it\cdot X}\right)=\int e^{it\cdot x} \, d\mu_X(x).</math>

As in the case of the "non-unitary angular frequency" convention above, the factor of 2{{pi}} appears in neither the normalizing constant nor the exponent. Unlike any of the conventions appearing above, this convention takes the opposite sign in the exponent.

== Computation methods ==

The appropriate computation method largely depends how the original mathematical function is represented and the desired form of the output function.

Since the fundamental definition of a Fourier transform is an integral, functions that can be expressed as [[closed-form expression]]s are commonly computed by working the integral analytically to yield a closed-form expression in the Fourier transform conjugate variable as the result. This is the method used to generate tables of Fourier transforms,<ref name="Zwillinger_2014">{{harvnb|Gradshteyn|Ryzhik|Geronimus|Tseytlin|2015}}.</ref> including those found in the table below ([[Fourier transform#Tables of important Fourier transforms]]).

Many computer algebra systems such as [[Matlab]] and [[Mathematica]] that are capable of [[symbolic integration]] are capable of computing Fourier transforms analytically. For example, to compute the Fourier transform of {{math|''f''&thinsp;(''t'') {{=}} cos(6π''t'') ''e''<sup>−π''t''<sup>2</sup></sup>}} one might enter the command {{code|integrate cos(6*pi*t) exp(−pi*t^2) exp(-i*2*pi*f*t) from -inf to inf}} into [[Wolfram Alpha]].

===Numerical integration of closed-form functions===
If the input function is in closed-form and the desired output function is a series of ordered pairs (for example a table of values from which a graph can be generated) over a specified domain, then the Fourier transform can be generated by [[numerical integration]] at each value of the Fourier conjugate variable (frequency, for example) for which a value of the output variable is desired.<ref>{{harvnb|Press|Flannery|Teukolsky|Vetterling|1992}}.</ref> Note that this method requires computing a separate numerical integration for each value of frequency for which a value of the Fourier transform is desired.<ref>{{harvnb|Bailey|Swarztrauber|1994}}.</ref><ref>{{harvnb|Lado|1971}}.</ref> The numerical integration approach works on a much broader class of functions than the analytic approach, because it yields results for functions that do not have closed form Fourier transform integrals.

===Numerical integration of a series of ordered pairs===
If the input function is a series of ordered pairs (for example, a time series from measuring an output variable repeatedly over a time interval) then the output function must also be a series of ordered pairs (for example, a complex number vs. frequency over a specified domain of frequencies), unless certain assumptions and approximations are made allowing the output function to be approximated by a closed-form expression. In the general case where the available input series of ordered pairs are assumed be samples representing a continuous function over an interval (amplitude vs. time, for example), the series of ordered pairs representing the desired output function can be obtained by numerical integration of the input data over the available interval at each value of the Fourier conjugate variable (frequency, for example) for which the value of the Fourier transform is desired.<ref>{{harvnb|Simonen|Olkkonen|1985}}.</ref>

Explicit numerical integration over the ordered pairs can yield the Fourier transform output value for any desired value of the conjugate Fourier transform variable (frequency, for example), so that a spectrum can be produced at any desired step size and over any desired variable range for accurate determination of amplitudes, frequencies, and phases corresponding to isolated peaks. Unlike limitations in DFT and FFT methods, explicit numerical integration can have any desired step size and compute the Fourier transform over any desired range of the conjugate Fourier transform variable (for example, frequency).

===Discrete Fourier transforms and fast Fourier transforms===
If the ordered pairs representing the original input function are equally spaced in their input variable (for example, equal time steps), then the Fourier transform is known as a [[discrete Fourier transform]] (DFT), which can be computed either by explicit numerical integration, by explicit evaluation of the DFT definition, or by [[fast Fourier transform]] (FFT) methods. In contrast to explicit integration of input data, use of the DFT and FFT methods produces Fourier transforms described by ordered pairs of step size equal to the reciprocal of the original sampling interval. For example, if the input data is sampled every 10 seconds, the output of DFT and FFT methods will have a 0.1&nbsp;Hz frequency spacing.

==Tables of important Fourier transforms==
The following tables record some closed-form Fourier transforms. For functions {{math|''f''&thinsp;(''x'')}}, {{math|''g''(''x'')}} and {{math|''h''(''x'')}} denote their Fourier transforms by {{math|''f̂''}}, {{math|''ĝ''}}, and {{math|''ĥ''}} respectively. Only the three most common conventions are included. It may be useful to notice that entry 105 gives a relationship between the Fourier transform of a function and the original function, which can be seen as relating the Fourier transform and its inverse.

===Functional relationships, one-dimensional===
The Fourier transforms in this table may be found in {{harvtxt|Erdélyi|1954}} or {{harvtxt|Kammler|2000|loc=appendix}}.
{| class="wikitable"
! !! Function !! Fourier transform <br /> unitary, ordinary frequency !! Fourier transform <br /> unitary, angular frequency !! Fourier transform <br /> non-unitary, angular frequency !!Remarks
|-
|
|<math> f(x)\,</math>
|<math>\begin{align} &\hat{f}(\xi) \\&= \int_{-\infty}^\infty f(x) e^{-2\pi i x\xi}\, dx \end{align}</math>
|<math>\begin{align} &\hat{f}(\omega) \\&= \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^\infty f(x) e^{-i \omega x}\, dx \end{align}</math>
|<math>\begin{align} &\hat{f}(\nu) \\&= \int_{-\infty}^\infty f(x) e^{-i \nu x}\, dx \end{align}</math>
|Definition
|-
| 101
|<math> a\cdot f(x) + b\cdot g(x)\,</math>
|<math> a\cdot \hat{f}(\xi) + b\cdot \hat{g}(\xi)\,</math>
|<math> a\cdot \hat{f}(\omega) + b\cdot \hat{g}(\omega)\,</math>
|<math> a\cdot \hat{f}(\nu) + b\cdot \hat{g}(\nu)\,</math>
|Linearity
|-
| 102
|<math> f(x - a)\,</math>
|<math> e^{-2\pi i a \xi} \hat{f}(\xi)\,</math>
|<math> e^{- i a \omega} \hat{f}(\omega)\,</math>
|<math> e^{- i a \nu} \hat{f}(\nu)\,</math>
|Shift in time domain
|-
| 103
|<math> f(x)e^{iax}\,</math>
|<math> \hat{f} \left(\xi - \frac{a}{2\pi}\right)\,</math>
|<math> \hat{f}(\omega - a)\,</math>
|<math> \hat{f}(\nu - a)\,</math>
|Shift in frequency domain, dual of 102
|-
| 104
|<math> f(a x)\,</math>
|<math> \frac{1}{|a|} \hat{f}\left( \frac{\xi}{a} \right)\,</math>
|<math> \frac{1}{|a|} \hat{f}\left( \frac{\omega}{a} \right)\,</math>
|<math> \frac{1}{|a|} \hat{f}\left( \frac{\nu}{a} \right)\,</math>
|Scaling in the time domain. If {{math|{{abs|''a''}}}} is large, then {{math|''f''&thinsp;(''ax'')}} is concentrated around 0 and<br><math> \frac{1}{|a|}\hat{f} \left( \frac{\omega}{a} \right)\,</math><br>spreads out and flattens.
|-
| 105
|<math> \hat{f}(x)\,</math>
|<math> f(-\xi)\,</math>
|<math> f(-\omega)\,</math>
|<math> 2\pi f(-\nu)\,</math>
|Duality. Here {{math|''f̂''}} needs to be calculated using the same method as Fourier transform column. Results from swapping "dummy" variables of {{mvar|x}} and {{mvar|ξ}} or {{mvar|ω}} or {{mvar|ν}}.
|-
| 106
|<math> \frac{d^n f(x)}{dx^n}\,</math>
|<math> (2\pi i\xi)^n \hat{f}(\xi)\,</math>
|<math> (i\omega)^n \hat{f}(\omega)\,</math>
|<math> (i\nu)^n \hat{f}(\nu)\,</math>
|
|-
| 107
|<math> x^n f(x)\,</math>
|<math> \left (\frac{i}{2\pi}\right)^n \frac{d^n \hat{f}(\xi)}{d\xi^n}\,</math>
|<math> i^n \frac{d^n \hat{f}(\omega)}{d\omega^n}</math>
|<math> i^n \frac{d^n \hat{f}(\nu)}{d\nu^n}</math>
|This is the dual of 106
|-
| 108
|<math> (f * g)(x)\,</math>
|<math> \hat{f}(\xi) \hat{g}(\xi)\,</math>
|<math> \sqrt{2\pi} \hat{f}(\omega) \hat{g}(\omega)\,</math>
|<math> \hat{f}(\nu) \hat{g}(\nu)\,</math>
|The notation {{math|''f'' ∗ ''g''}} denotes the [[convolution]] of {{mvar|f}} and {{mvar|g}} — this rule is the [[convolution theorem]]
|-
| 109
|<math> f(x) g(x)\,</math>
|<math> \left(\hat{f} * \hat{g}\right)(\xi)\,</math>
|<math> \frac{1}\sqrt{2\pi}\left(\hat{f} * \hat{g}\right)(\omega)\,</math>
|<math> \frac{1}{2\pi}\left(\hat{f} * \hat{g}\right)(\nu)\,</math>
|This is the dual of 108
|-
| 110
|For {{math|''f''&thinsp;(''x'')}} purely real
|<math> \hat{f}(-\xi) = \overline{\hat{f}(\xi)}\,</math>
|<math> \hat{f}(-\omega) = \overline{\hat{f}(\omega)}\,</math>
|<math> \hat{f}(-\nu) = \overline{\hat{f}(\nu)}\,</math>
|Hermitian symmetry. {{math|{{overline|''z''}}}} indicates the [[complex conjugate]].
|-
| 111
|For {{math|''f''&thinsp;(''x'')}} purely real and [[even function|even]]
| colspan=3 align=center |{{math|''f̂''&thinsp;(''ξ'')}}, {{math|''f̂''&thinsp;(''ω'')}} and {{math|''f̂''&thinsp;(''ν'')}} are purely real [[even function]]s.
|
|-
| 112
|For {{math|''f''&thinsp;(''x'')}} purely real and [[odd function|odd]]
| colspan=3 align=center |{{math|''f̂''&thinsp;(''ξ'')}}, {{math|''f̂''&thinsp;(''ω'')}} and {{math|''f̂''&thinsp;(''ν'')}} are purely [[imaginary number|imaginary]] [[odd function]]s.
|
|-
| 113
|For {{math|''f''&thinsp;(''x'')}} purely imaginary
|<math> \hat{f}(-\xi) = -\overline{\hat{f}(\xi)}\,</math>
|<math> \hat{f}(-\omega) = -\overline{\hat{f}(\omega)}\,</math>
|<math> \hat{f}(-\nu) = -\overline{\hat{f}(\nu)}\,</math>
|{{math|{{overline|''z''}}}} indicates the [[complex conjugate]].
|-
| 114
| <math> \overline{f(x)}</math>|| <math> \overline{\hat{f}(-\xi)}</math> || <math> \overline{\hat{f}(-\omega)}</math> || <math> \overline{\hat{f}(-\nu)}</math> || [[Complex conjugate|Complex conjugation]], generalization of 110 and 113
|-
|115
|<math> f(x) \cos (a x)</math>
|<math> \frac{ \hat{f}\left(\xi - \frac{a}{2\pi}\right)+\hat{f}\left(\xi+\frac{a}{2\pi}\right)}{2}</math>
|<math> \frac{\hat{f}(\omega-a)+\hat{f}(\omega+a)}{2}\,</math>
|<math> \frac{\hat{f}(\nu-a)+\hat{f}(\nu+a)}{2}</math>
|This follows from rules 101 and 103 using [[Euler's formula]]:<br><math>\cos(a x) = \frac{e^{i a x} + e^{-i a x}}{2}.</math>
|-
|116
|<math> f(x)\sin( ax)</math>
|<math> \frac{\hat{f}\left(\xi-\frac{a}{2\pi}\right)-\hat{f}\left(\xi+\frac{a}{2\pi}\right)}{2i}</math>
|<math> \frac{\hat{f}(\omega-a)-\hat{f}(\omega+a)}{2i}</math>
|<math> \frac{\hat{f}(\nu-a)-\hat{f}(\nu+a)}{2i}</math>
|This follows from 101 and 103 using [[Euler's formula]]:<br><math>\sin(a x) = \frac{e^{i a x} - e^{-i a x}}{2i}.</math>
|}

===Square-integrable functions, one-dimensional===
The Fourier transforms in this table may be found in {{harvtxt|Campbell|Foster|1948}}, {{harvtxt|Erdélyi|1954}}, or {{harvtxt|Kammler|2000|loc=appendix}}.
{| class="wikitable"
! !! Function !! Fourier transform <br /> unitary, ordinary frequency !! Fourier transform <br /> unitary, angular frequency !! Fourier transform <br /> non-unitary, angular frequency !! Remarks
|-
|
|<math> f(x)\,</math>
|<math>\begin{align} &\hat{f}(\xi) \\&= \int_{-\infty}^\infty f(x) e^{-2\pi i x\xi}\, dx \end{align}</math>
|<math>\begin{align} &\hat{f}(\omega) \\&= \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^\infty f(x) e^{-i \omega x}\, dx \end{align}</math>
|<math>\begin{align} &\hat{f}(\nu) \\&= \int_{-\infty}^\infty f(x) e^{-i \nu x}\, dx \end{align}</math>
|
|-
|{{anchor|rect}} 201
|<math> \operatorname{rect}(a x) \,</math>
|<math> \frac{1}{|a|}\cdot \operatorname{sinc}\left(\frac{\xi}{a}\right)</math>
|<math> \frac{1}{\sqrt{2 \pi a^2}}\cdot \operatorname{sinc}\left(\frac{\omega}{2\pi a}\right)</math>
|<math> \frac{1}{|a|}\cdot \operatorname{sinc}\left(\frac{\nu}{2\pi a}\right)</math>
|The [[rectangular function|rectangular pulse]] and the ''normalized'' [[sinc function]], here defined as {{math|sinc(''x'') {{=}} {{sfrac|sin(π''x'')|π''x''}}}}
|-
| 202
|<math> \operatorname{sinc}(a x)\,</math>
|<math> \frac{1}{|a|}\cdot \operatorname{rect}\left(\frac{\xi}{a} \right)\,</math>
|<math> \frac{1}{\sqrt{2\pi a^2}}\cdot \operatorname{rect}\left(\frac{\omega}{2 \pi a}\right)</math>
|<math> \frac{1}{|a|}\cdot \operatorname{rect}\left(\frac{\nu}{2 \pi a}\right)</math>
|Dual of rule 201. The [[rectangular function]] is an ideal [[low-pass filter]], and the [[sinc function]] is the [[Anticausal system|non-causal]] impulse response of such a filter. The [[sinc function]] is defined here as {{math|sinc(''x'') {{=}} {{sfrac|sin(π''x'')|π''x''}}}}
|-
| 203
|<math> \operatorname{sinc}^2 (a x)</math>
|<math> \frac{1}{|a|}\cdot \operatorname{tri} \left( \frac{\xi}{a} \right) </math>
|<math> \frac{1}{\sqrt{2\pi a^2}}\cdot \operatorname{tri} \left( \frac{\omega}{2\pi a} \right) </math>
|<math> \frac{1}{|a|}\cdot \operatorname{tri} \left( \frac{\nu}{2\pi a} \right) </math>
| The function {{math|tri(''x'')}} is the [[triangular function]]
|-
| 204
|<math> \operatorname{tri} (a x)</math>
|<math> \frac{1}{|a|}\cdot \operatorname{sinc}^2 \left( \frac{\xi}{a} \right) \,</math>
|<math> \frac{1}{\sqrt{2\pi a^2}} \cdot \operatorname{sinc}^2 \left( \frac{\omega}{2\pi a} \right) </math>
|<math> \frac{1}{|a|} \cdot \operatorname{sinc}^2 \left( \frac{\nu}{2\pi a} \right) </math>
| Dual of rule 203.
|-
| 205
|<math> e^{- a x} u(x) \,</math>
|<math> \frac{1}{a + 2 \pi i \xi}</math>
|<math> \frac{1}{\sqrt{2 \pi} (a + i \omega)}</math>
|<math> \frac{1}{a + i \nu}</math>
|The function {{math|''u''(''x'')}} is the [[Heaviside step function|Heaviside unit step function]] and {{math|''a'' > 0}}.
|-
| 206
|<math> e^{-\alpha x^2}\,</math>
|<math> \sqrt{\frac{\pi}{\alpha}}\cdot e^{-\frac{(\pi \xi)^2}{\alpha}}</math>
|<math> \frac{1}{\sqrt{2 \alpha}}\cdot e^{-\frac{\omega^2}{4 \alpha}}</math>
|<math> \sqrt{\frac{\pi}{\alpha}}\cdot e^{-\frac{\nu^2}{4 \alpha}}</math>
|This shows that, for the unitary Fourier transforms, the [[Gaussian function]] {{math|''e''<sup>−''αx''<sup>2</sup></sup>}} is its own Fourier transform for some choice of {{mvar|α}}. For this to be integrable we must have {{math|Re(''α'') > 0}}.
|-
| 207
|<math> \operatorname{e}^{-a|x|} \,</math>
|<math> \frac{2 a}{a^2 + 4 \pi^2 \xi^2} </math>
|<math> \sqrt{\frac{2}{\pi}} \cdot \frac{a}{a^2 + \omega^2} </math>
|<math> \frac{2a}{a^2 + \nu^{2}} </math>
|For {{math|Re(''a'') > 0}}. That is, the Fourier transform of a [[Laplace distribution|two-sided decaying exponential function]] is a [[Lorentzian function]].
|-
| 208
|<math> \operatorname{sech}(a x) \,</math>
|<math> \frac{\pi}{a} \operatorname{sech} \left( \frac{\pi^2}{ a} \xi \right)</math>
|<math> \frac{1}{a}\sqrt{\frac{\pi}{2}}\operatorname{sech}\left( \frac{\pi}{2 a} \omega \right)</math>
|<math> \frac{\pi}{a}\operatorname{sech}\left( \frac{\pi}{2 a} \nu \right)</math>
|[[Hyperbolic function|Hyperbolic secant]] is its own Fourier transform
|-
| 209
|<math> e^{-\frac{a^2 x^2}2} H_n(a x)\,</math>
|<math> \frac{\sqrt{2\pi}(-i)^n}{a} e^{-\frac{2\pi^2\xi^2}{a^2}} H_n\left(\frac{2\pi\xi}a\right)</math>
|<math> \frac{(-i)^n}{a} e^{-\frac{\omega^2}{2 a^2}} H_n\left(\frac \omega a\right)</math>
|<math> \frac{(-i)^n \sqrt{2\pi}}{a} e^{-\frac{\nu^2}{2 a^2}} H_n\left(\frac \nu a \right)</math>
|{{math|''H<sub>n</sub>''}} is the {{mvar|n}}th-order [[Hermite polynomial]]. If {{math|''a'' {{=}} 1}} then the Gauss–Hermite functions are [[eigenfunction]]s of the Fourier transform operator. For a derivation, see [[Hermite polynomials##Hermite functions as eigenfunctions of the Fourier transform|Hermite polynomial]]. The formula reduces to 206 for {{math|''n'' {{=}} 0}}.
|}

===Distributions, one-dimensional===
The Fourier transforms in this table may be found in {{harvtxt|Erdélyi|1954}} or {{harvtxt|Kammler|2000|loc=appendix}}.
{| class="wikitable"
! !! Function !! Fourier transform <br /> unitary, ordinary frequency !! Fourier transform <br /> unitary, angular frequency !! Fourier transform <br /> non-unitary, angular frequency !! Remarks
|-
|
|<math> f(x)\,</math>
|<math>\begin{align} &\hat{f}(\xi) = \int_{-\infty}^\infty f(x) e^{-2\pi i x\xi}\, dx \end{align}</math>
|<math>\begin{align} &\hat{f}(\omega) = \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^\infty f(x) e^{-i \omega x}\, dx \end{align}</math>
|<math>\begin{align} &\hat{f}(\nu)= \int_{-\infty}^\infty f(x) e^{-i \nu x}\, dx \end{align}</math>
|
|-
| 301
|<math> 1</math>
|<math> \delta(\xi)</math>
|<math> \sqrt{2\pi}\cdot \delta(\omega)</math>
|<math> 2\pi\delta(\nu)</math>
|The distribution {{math|''δ''(''ξ'')}} denotes the [[Dirac delta function]].
|-
| 302
|<math> \delta(x)\,</math>
|<math> 1</math>
|<math> \frac{1}{\sqrt{2\pi}}\,</math>
|<math> 1</math>
|Dual of rule 301.
|-
| 303
|<math> e^{i a x}</math>
|<math> \delta\left(\xi - \frac{a}{2\pi}\right)</math>
|<math> \sqrt{2 \pi}\cdot \delta(\omega - a)</math>
|<math> 2 \pi\delta(\nu - a)</math>
|This follows from 103 and 301.
|-
| 304
|<math> \cos (a x)</math>
|<math> \frac{ \delta\left(\xi - \frac{a}{2\pi}\right)+\delta\left(\xi+\frac{a}{2\pi}\right)}{2}</math>
|<math> \sqrt{2 \pi}\cdot\frac{\delta(\omega-a)+\delta(\omega+a)}{2}\,</math>
|<math> \pi\left(\delta(\nu-a)+\delta(\nu+a)\right)</math>
|This follows from rules 101 and 303 using [[Euler's formula]]:<br><math>\cos(a x) = \frac{e^{i a x} + e^{-i a x}}{2}.</math>
|-
| 305
|<math> \sin( ax)</math>
|<math> \frac{\delta\left(\xi-\frac{a}{2\pi}\right)-\delta\left(\xi+\frac{a}{2\pi}\right)}{2i}</math>
|<math> \sqrt{2 \pi}\cdot\frac{\delta(\omega-a)-\delta(\omega+a)}{2i}</math>
|<math> -i\pi\bigl(\delta(\nu-a)-\delta(\nu+a)\bigr)</math>
|This follows from 101 and 303 using<br><math>\sin(a x) = \frac{e^{i a x} - e^{-i a x}}{2i}.</math>
|-
| 306
|<math> \cos \left( a x^2 \right) </math>
|<math> \sqrt{\frac{\pi}{a}} \cos \left( \frac{\pi^2 \xi^2}{a} - \frac{\pi}{4} \right) </math>
|<math> \frac{1}{\sqrt{2 a}} \cos \left( \frac{\omega^2}{4 a} - \frac{\pi}{4} \right) </math>
|<math> \sqrt{\frac{\pi}{a}} \cos \left( \frac{\nu^2}{4a} - \frac{\pi}{4} \right) </math>
|
|-
| 307
|<math> \sin \left( a x^2 \right) \,</math>
|<math> - \sqrt{\frac{\pi}{a}} \sin \left( \frac{\pi^2 \xi^2}{a} - \frac{\pi}{4} \right) </math>
|<math> \frac{-1}{\sqrt{2 a}} \sin \left( \frac{\omega^2}{4 a} - \frac{\pi}{4} \right) </math>
|<math> -\sqrt{\frac{\pi}{a}}\sin \left( \frac{\nu^2}{4a} - \frac{\pi}{4} \right)</math>
|
|-
| 308
|<math> x^n\,</math>
|<math> \left(\frac{i}{2\pi}\right)^n \delta^{(n)} (\xi)\,</math>
|<math> i^n \sqrt{2\pi} \delta^{(n)} (\omega)\,</math>
|<math> 2\pi i^n\delta^{(n)} (\nu)\,</math>
|Here, {{mvar|n}} is a [[natural number]] and {{math|''δ''{{isup|(''n'')}}(''ξ'')}} is the {{mvar|n}}th distribution derivative of the Dirac delta function. This rule follows from rules 107 and 301. Combining this rule with 101, we can transform all [[polynomial]]s.
|-
|
|<math> \delta^{(n)}(x)\,</math>
|<math> (2\pi i\xi)^n\,</math>
|<math> \frac{(i\omega)^n}{\sqrt{2\pi}} \,</math>
|<math> (i\nu)^n\,</math>
|Dual of rule 308. {{math|''δ''{{isup|(''n'')}}(''ξ'')}} is the {{mvar|n}}th distribution derivative of the Dirac delta function. This rule follows from 106 and 302.
|-
| 309
|<math> \frac{1}{x}</math>
|<math> -i\pi\sgn(\xi)</math>
|<math> -i\sqrt{\frac{\pi}{2}}\sgn(\omega)</math>
|<math> -i\pi\sgn(\nu)</math>
|Here {{math|sgn(''ξ'')}} is the [[sign function]]. Note that {{math|{{sfrac|1|''x''}}}} is not a distribution. It is necessary to use the [[Cauchy principal value]] when testing against [[Schwartz functions]]. This rule is useful in studying the [[Hilbert transform]].
|-
| 310
|<math>\begin{align} &\frac{1}{x^n} \\ &:= \frac{(-1)^{n-1}}{(n-1)!}\frac{d^n}{dx^n}\log |x| \end{align}</math>
|<math> -i\pi \frac{(-2\pi i\xi)^{n-1}}{(n-1)!} \sgn(\xi)</math>
|<math> -i\sqrt{\frac{\pi}{2}}\cdot \frac{(-i\omega)^{n-1}}{(n-1)!}\sgn(\omega)</math>
|<math> -i\pi \frac{(-i\nu)^{n-1}}{(n-1)!}\sgn(\nu)</math>
|{{math|{{sfrac|1|''x''<sup>''n''</sup>}}}} is the [[homogeneous distribution]] defined by the distributional derivative<br><math>\frac{(-1)^{n-1}}{(n-1)!}\frac{d^n}{dx^n}\log|x|</math>
|-
| 311
|<math> |x|^\alpha\,</math>
|<math> -\frac{2\sin\left(\frac{\pi\alpha}{2}\right)\Gamma(\alpha+1)}{|2\pi\xi|^{\alpha+1}}</math>
|<math> \frac{-2}{\sqrt{2\pi}}\cdot \frac{\sin\left(\frac{\pi\alpha}{2}\right)\Gamma(\alpha+1)}{|\omega|^{\alpha+1}} </math>
|<math> -\frac{2\sin\left(\frac{\pi\alpha}{2}\right)\Gamma(\alpha+1)}{|\nu|^{\alpha+1}} </math>
|This formula is valid for {{math|0 > ''α'' > −1}}. For {{math|''α'' > 0}} some singular terms arise at the origin that can be found by differentiating 318. If {{math|Re ''α'' > −1}}, then {{math|{{abs|''x''}}<sup>''α''</sup>}} is a locally integrable function, and so a tempered distribution. The function {{math|''α'' ↦ {{abs|''x''}}<sup>''α''</sup>}} is a holomorphic function from the right half-plane to the space of tempered distributions. It admits a unique meromorphic extension to a tempered distribution, also denoted {{math|{{abs|''x''}}<sup>''α''</sup>}} for {{math|''α'' ≠ −2, −4,...}} (See [[homogeneous distribution]].)
|-
| <!-- Should we call it 311a ? Doesn't necessarily need a number, because it's a special case. -->
|<math> \frac{1}{\sqrt{|x|}} \,</math>
|<math> \frac{1}{\sqrt{|\xi|}} </math>
|<math> \frac{1}{\sqrt{|\omega|}}</math>
|<math> \frac{\sqrt{2\pi}}{\sqrt{|\nu|}} </math>
| Special case of 311.
|-
| 312
|<math> \sgn(x)</math>
|<math> \frac{1}{i\pi \xi}</math>
|<math> \sqrt{\frac{2}{\pi}} \frac{1}{i\omega } </math>
|<math> \frac{2}{i\nu }</math>
|The dual of rule 309. This time the Fourier transforms need to be considered as a [[Cauchy principal value]].
|-
| 313
|<math> u(x)</math>
|<math> \frac{1}{2}\left(\frac{1}{i \pi \xi} + \delta(\xi)\right)</math>
|<math> \sqrt{\frac{\pi}{2}} \left( \frac{1}{i \pi \omega} + \delta(\omega)\right)</math>
|<math> \pi\left( \frac{1}{i \pi \nu} + \delta(\nu)\right)</math>
|The function {{math|''u''(''x'')}} is the Heaviside [[Heaviside step function|unit step function]]; this follows from rules 101, 301, and 312.
|-
| 314
|<math> \sum_{n=-\infty}^{\infty} \delta (x - n T)</math>
|<math> \frac{1}{T} \sum_{k=-\infty}^{\infty} \delta \left( \xi -\frac{k }{T}\right)</math>
|<math> \frac{\sqrt{2\pi }}{T}\sum_{k=-\infty}^{\infty} \delta \left( \omega -\frac{2\pi k}{T}\right)</math>
|<math> \frac{2\pi}{T}\sum_{k=-\infty}^{\infty} \delta \left( \nu -\frac{2\pi k}{T}\right)</math>
|This function is known as the [[Dirac comb]] function. This result can be derived from 302 and 102, together with the fact that<br><math>\sum_{n=-\infty}^{\infty} e^{inx}= 2\pi\sum_{k=-\infty}^{\infty} \delta(x+2\pi k)</math><br>as distributions.
|-
| 315
|<math> J_0 (x)</math>
|<math> \frac{2\, \operatorname{rect}(\pi\xi)}{\sqrt{1 - 4 \pi^2 \xi^2}} </math>
|<math> \sqrt{\frac{2}{\pi}} \cdot \frac{\operatorname{rect}\left(  \frac{\omega}{2} \right)}{\sqrt{1 - \omega^2}} </math>
|<math> \frac{2\,\operatorname{rect}\left(\frac{\nu}{2} \right)}{\sqrt{1 - \nu^2}}</math>
| The function {{math|''J''<sub>0</sub>(''x'')}} is the zeroth order [[Bessel function]] of first kind.
|-
| 316
|<math> J_n (x)</math>
|<math> \frac{2 (-i)^n T_n (2 \pi \xi) \operatorname{rect}(\pi \xi)}{\sqrt{1 - 4 \pi^2 \xi^2}} </math>
|<math> \sqrt{\frac{2}{\pi}} \frac{ (-i)^n T_n (\omega) \operatorname{rect} \left( \frac{\omega}{2} \right)}{\sqrt{1 - \omega^2}} </math>
|<math> \frac{2(-i)^n T_n (\nu) \operatorname{rect} \left( \frac{\nu}{2} \right)}{\sqrt{1 - \nu^2}} </math>
| This is a generalization of 315. The function {{math|''J<sub>n</sub>''(''x'')}} is the {{mvar|n}}th order [[Bessel function]] of first kind. The function {{math|''T<sub>n</sub>''(''x'')}} is the [[Chebyshev polynomials|Chebyshev polynomial of the first kind]].
|-
| 317
|<math> \log \left| x \right|</math>
|<math> -\frac{1}{2} \frac{1}{\left| \xi \right|} - \gamma \delta \left( \xi \right) </math>
|<math> -\frac{\sqrt\frac{\pi}{2}}{\left| \omega \right|} - \sqrt{2 \pi} \gamma \delta \left( \omega \right) </math>
|<math> -\frac{\pi}{\left| \nu \right|} - 2 \pi \gamma \delta \left( \nu \right) </math>
|{{mvar|γ}} is the [[Euler–Mascheroni constant]].
|-
| 318
|<math> \left( \mp ix \right)^{-\alpha}</math>
|<math> \frac{\left(2\pi\right)^\alpha}{\Gamma\left(\alpha\right)}u\left(\pm \xi \right)\left(\pm \xi \right)^{\alpha-1} </math>
|<math> \frac{\sqrt{2\pi}}{\Gamma\left(\alpha\right)}u\left(\pm\omega\right)\left(\pm\omega\right)^{\alpha-1} </math>
|<math> \frac{2\pi}{\Gamma\left(\alpha\right)}u\left(\pm\nu\right)\left(\pm\nu\right)^{\alpha-1} </math>
|This formula is valid for {{math|1 > ''α'' > 0}}. Use differentiation to derive formula for higher exponents. {{mvar|u}} is the Heaviside function.
|}

===Two-dimensional functions===
{| class="wikitable"
! !! Function !! Fourier transform <br /> unitary, ordinary frequency !! Fourier transform <br /> unitary, angular frequency !! Fourier transform <br /> non-unitary, angular frequency !! Remarks
|-
|400
|<math> f(x,y)</math>
|<math>\begin{align}& \hat{f}(\xi_x, \xi_y) \\ &= \iint f(x,y) e^{-2\pi i(\xi_x x+\xi_y y)}\,dx\,dy \end{align}</math>
|<math>\begin{align}& \hat{f}(\omega_x,\omega_y) \\ &= \frac{1}{2 \pi} \iint f(x,y) e^{-i (\omega_x x +\omega_y y)}\, dx\,dy  \end{align}</math>
|<math>\begin{align}& \hat{f}(\nu_x,\nu_y) \\ &= \iint f(x,y) e^{-i(\nu_x x+\nu_y y)}\, dx\,dy  \end{align}</math>
|The variables {{mvar|ξ<sub>x</sub>}}, {{mvar|ξ<sub>y</sub>}}, {{mvar|ω<sub>x</sub>}}, {{mvar|ω<sub>y</sub>}}, {{mvar|ν<sub>x</sub>}}, {{mvar|ν<sub>y</sub>}} are real numbers. The integrals are taken over the entire plane.
|-
|401
|<math> e^{-\pi\left(a^2x^2+b^2y^2\right)}</math>
|<math> \frac{1}{|ab|} e^{-\pi\left(\frac{\xi_x^2}{a^2} + \frac{\xi_y^2}{b^2}\right)}</math>
|<math> \frac{1}{2\pi\cdot|ab|} e^{-\frac{1}{4\pi}\left(\frac{\omega_x^2}{a^2} + \frac{\omega_y^2}{b^2}\right)}</math>
|<math> \frac{1}{|ab|} e^{-\frac{1}{4\pi}\left(\frac{\nu_x^2}{a^2} + \frac{\nu_y^2}{b^2}\right)}</math>
|Both functions are Gaussians, which may not have unit volume.
|-
|402
|<math> \operatorname{circ}\left(\sqrt{x^2+y^2}\right)</math>
|<math> \frac{J_1\left(2 \pi \sqrt{\xi_x^2+\xi_y^2}\right)}{\sqrt{\xi_x^2+\xi_y^2}}</math>
|<math> \frac{J_1\left(\sqrt{\omega_x^2+\omega_y^2}\right)}{\sqrt{\omega_x^2+\omega_y^2}}</math>
|<math> \frac{2\pi J_1\left(\sqrt{\nu_x^2+\nu_y^2}\right)}{\sqrt{\nu_x^2+\nu_y^2}}</math>
|The function is defined by {{math|circ(''r'') {{=}} 1}} for {{math|0 ≤ ''r'' ≤ 1}}, and is 0 otherwise. The result is the amplitude distribution of the [[Airy disk]], and is expressed using {{math|''J''<sub>1</sub>}} (the order-1 [[Bessel function]] of the first kind).<ref>{{harvnb|Stein|Weiss|1971|loc=Thm. IV.3.3}}.</ref>
|-
|403
|<math> \frac{i}{x+i y}</math>
|<math> \frac{1}{\xi_x+i\xi_y}</math>
|<math> \frac{1}{\omega_x+i\omega_y}</math>
|<math> \frac{2\pi}{\nu_x+i\nu_y}</math>
|<!--This formula was used in constructing the ground state wavefunction of two-dimensional <math> p_x+ip_y</math> superconductors <ref>Phys. Rev. B 97 (10), 104501 (2018) </ref>-->
|}

===Formulas for general {{math|''n''}}-dimensional functions===
{| class="wikitable"
! !! Function !! Fourier transform <br /> unitary, ordinary frequency !! Fourier transform <br /> unitary, angular frequency !! Fourier transform <br /> non-unitary, angular frequency !! Remarks
|-
|500
|<math> f(\mathbf x)\,</math>
|<math>\begin{align} &\hat{f}(\boldsymbol \xi) \\ &= \int_{\mathbb{R}^n}f(\mathbf x) e^{-2\pi i \mathbf x \cdot \boldsymbol \xi }\, d \mathbf x \end{align}</math>
|<math>\begin{align} &\hat{f}(\boldsymbol \omega) \\ &= \frac{1}{{(2 \pi)}^\frac{n}{2}} \int_{\mathbb{R}^n} f(\mathbf x) e^{-i \boldsymbol \omega \cdot \mathbf x}\, d \mathbf x \end{align}</math>
|<math>\begin{align} &\hat{f}(\boldsymbol \nu) \\ &= \int_{\mathbb{R}^n}f(\mathbf x) e^{-i \mathbf x \cdot \boldsymbol \nu }\, d \mathbf x \end{align}</math>
|
|-
|501
|<math> \chi_{[0,1]}(|\mathbf x|)\left(1-|\mathbf x|^2\right)^\delta</math>
|<math> \pi^{-\delta}\Gamma(\delta+1)|\boldsymbol \xi|^{-\frac{n}{2}-\delta} J_{\frac{n}{2}+\delta}(2\pi|\boldsymbol \xi|)</math>
|<math> 2^{-\delta}\Gamma(\delta+1)\left|\boldsymbol \omega\right|^{-\frac{n}{2}-\delta} J_{\frac{n}{2}+\delta}(|\boldsymbol \omega|)</math>
|<math> \pi^{-\delta}\Gamma(\delta+1)\left|\frac{\boldsymbol \nu}{2\pi}\right|^{-\frac{n}{2}-\delta} J_{\frac{n}{2}+\delta}(|\boldsymbol \nu|)</math>
|The function {{math|''χ''<sub>[0, 1]</sub>}} is the [[indicator function]] of the interval {{math|[0, 1]}}. The function {{math|Γ(''x'')}} is the gamma function. The function {{math|''J''<sub>{{sfrac|''n''|2}} + ''δ''</sub>}} is a Bessel function of the first kind, with order {{math|{{sfrac|''n''|2}} + ''δ''}}. Taking {{math|''n'' {{=}} 2}} and {{math|''δ'' {{=}} 0}} produces 402.<ref>{{harvnb|Stein|Weiss|1971|loc=Thm. 4.15}}.</ref>
|-
|502
|<math> |\mathbf x|^{-\alpha}, \quad 0 < \operatorname{Re} \alpha < n.</math>
|<math> \frac{(2\pi)^{\alpha}}{c_{n, \alpha}} |\boldsymbol \xi|^{-(n - \alpha)}</math>
|<math> \frac{(2\pi)^{\frac{n}{2}}}{c_{n, \alpha}} |\boldsymbol \omega|^{-(n - \alpha)}</math>
|<math> \frac{(2\pi)^{n}}{c_{n, \alpha}} |\boldsymbol \nu|^{-(n - \alpha)}</math>
|See [[Riesz potential]] where the constant is given by<br><math>c_{n, \alpha} = \pi^\frac{n}{2} 2^\alpha \frac{\Gamma\left(\frac{\alpha}{2}\right)}{\Gamma\left(\frac{n - \alpha}{2}\right)}.</math><br>The formula also holds for all {{math|''α'' ≠ −''n'', −''n'' − 1, ...}} by analytic continuation, but then the function and its Fourier transforms need to be understood as suitably regularized tempered distributions. See [[homogeneous distribution]].<ref group="remark">In {{harvnb|Gelfand|Shilov|1964|p=363}}, with the non-unitary conventions of this table, the transform of <math>|\mathbf x|^\lambda</math> is given to be<br /> <math>2^{\lambda+n}\pi^{\tfrac12 n}\frac{\Gamma\left(\frac{\lambda+n}{2}\right)}{\Gamma\left(-\frac{\lambda}{2}\right)}|\boldsymbol\nu|^{-\lambda-n}</math> <br />from which this follows, with <math>\lambda=-\alpha</math>.</ref>
|-
|503
|<math> \frac{1}{\left|\boldsymbol \sigma\right|\left(2\pi\right)^\frac{n}{2}} e^{-\frac{1}{2} \mathbf x^{\mathrm T} \boldsymbol \sigma^{-\mathrm T} \boldsymbol \sigma^{-1} \mathbf x}</math>
|<math> e^{-2\pi^2 \boldsymbol \xi^{\mathrm T} \boldsymbol \sigma \boldsymbol \sigma^{\mathrm T} \boldsymbol \xi} </math>
|<math> (2\pi)^{-\frac{n}{2}} e^{-\frac{1}{2} \boldsymbol \omega^{\mathrm T} \boldsymbol \sigma \boldsymbol \sigma^{\mathrm T} \boldsymbol \omega} </math>
|<math> e^{-\frac{1}{2} \boldsymbol \nu^{\mathrm T} \boldsymbol \sigma \boldsymbol \sigma^{\mathrm T} \boldsymbol \nu} </math>
|This is the formula for a [[multivariate normal distribution]] normalized to 1 with a mean of 0. Bold variables are vectors or matrices. Following the notation of the aforementioned page, {{math|'''Σ''' {{=}} '''σ''' '''σ'''<sup>T</sup>}} and {{math|'''Σ'''<sup>−1</sup> {{=}} '''σ'''<sup>−T</sup> '''σ'''<sup>−1</sup>}}
|-
|504
|<math> e^{-2\pi\alpha|\mathbf x|}</math>
| <math>\frac{c_n\alpha}{\left(\alpha^2+|\boldsymbol{\xi}|^2\right)^\frac{n+1}{2}}</math>
|<math>\frac{c_n (2\pi)^{\frac{n+2}{2}} \alpha}{\left(4\pi^2\alpha^2+|\boldsymbol{\omega}|^2\right)^\frac{n+1}{2}}</math>
|<math>\frac{c_n (2\pi)^{n+1} \alpha}{\left(4\pi^2\alpha^2+|\boldsymbol{\nu}|^2\right)^\frac{n+1}{2}}</math>
|Here<ref>{{harvnb|Stein|Weiss|1971|p=6}}.</ref><br><math>c_n=\frac{\Gamma\left(\frac{n+1}{2}\right)}{\pi^\frac{n+1}{2}}.</math>
|}

==See also==
{{div col|colwidth=22em}}
*[[Analog signal processing]]
*[[Beevers–Lipson strip]]
*[[Discrete Fourier transform]]
**[[DFT matrix]]
*[[Fast Fourier transform]]
*[[Fourier integral operator]]
*[[Fourier inversion theorem]]
*[[Fourier multiplier]]
*[[Fourier series]]
*[[Fourier sine transform]]
*[[Fourier–Deligne transform]]
*[[Fourier–Mukai transform]]
*[[Fractional Fourier transform]]
*[[Indirect Fourier transform]] 
*[[Integral transform]]
**[[Hankel transform]]
**[[Hartley transform]]
*[[Laplace transform]]
*[[Linear canonical transform]]
*[[Mellin transform]]
*[[Multidimensional transform]]
*[[NGC 4622]], especially the image NGC 4622 Fourier transform {{math|''m'' {{=}} 2}}.
*[[Nonlocal operator]]
*[[Short-time Fourier transform]]
*[[Spectral density]]
**[[Spectral density estimation]]
*[[Symbolic integration]]
*[[Time stretch dispersive Fourier transform]]
*[[Transform (mathematics)]]
{{div col end}}

== Remarks==
{{reflist|group="remark"}}

== Notes ==
{{reflist|22em}}

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{{refend}}

==External links==
*[https://www.encyclopediaofmath.org/index.php/Fourier_transform Encyclopedia of Mathematics]
*{{MathWorld | urlname = FourierTransform | title = Fourier Transform}}

{{Authority control}}

{{DEFAULTSORT:Fourier Transform}}
[[Category:Fourier analysis]]
[[Category:Integral transforms]]
[[Category:Unitary operators]]
[[Category:Joseph Fourier]]
[[Category:Mathematical physics]]