View source for Square root

{{short description|Number whose square is a given number}}
{{Redirect|Square roots|other uses|Square Roots (disambiguation)}}

[[File:Nuvola apps edu mathematics blue-p.svg|thumb|right|168px|The mathematical expression "The (principal) square root of {{mvar|x}}"]]
[[Image:Five Squared.svg|thumb|right|168px<!-- at 160px and 200px lines render with unequal widths -->|For example, {{math|{{sqrt|25}} {{=}} 5}}, since {{math|25 {{=}} 5 &sdot; 5}}, or {{math|5<sup>2</sup>}} (5 squared).]]

In [[mathematics]], a '''square root''' of a number {{math|''x''}} is a number {{math|''y''}} such that {{math|1=''y''<sup>2</sup> = ''x''}}; in other words, a number {{math|''y''}} whose ''[[square (algebra)|square]]'' (the result of multiplying the number by itself, or {{math|''y''}} &sdot; {{math|''y''}}) is {{math|''x''}}.<ref>Gel'fand, [https://books.google.com/books?id=Z9z7iliyFD0C&pg=PA120 p. 120] {{webarchive|url=https://web.archive.org/web/20160902151740/https://books.google.com/books?id=Z9z7iliyFD0C&pg=PA120 |date=2016-09-02 }}
</ref> For example, 4 and −4 are square roots of 16 because {{math|1=4<sup>2</sup> = (−4)<sup>2</sup> = 16}}.
Every nonnegative [[real number]] {{math|''x''}} has a unique nonnegative square root, called the ''principal square root'', which is denoted by {{math|{{sqrt|''x''}}}}, where {{math|{{sqrt|&nbsp;}}}} is called the ''[[radical sign]]'' or ''radix''. For example, the principal square root of 9 is 3, which is denoted by {{math|1={{sqrt|9}} = 3}}, because {{math|1=3<sup>2</sup> = 3 &sdot; 3 = 9}} and 3 is nonnegative.  The term (or number) whose square root is being considered is known as the ''radicand''. The radicand is the number or expression underneath the radical sign, in this example 9.

Every [[positive number]] {{math|''x''}} has two square roots: {{math|{{sqrt|''x''}}}}, which is positive, and {{math|−{{sqrt|''x''}}}}, which is negative. Together, these two roots are denoted as {{math|±{{sqrt|''x''}}}} (see [[Plus-minus_sign|±&nbsp;shorthand]]). Although the principal square root of a positive number is only one of its two square roots, the designation "''the'' square root" is often used to refer to the ''principal square root''. For positive {{math|''x''}}, the principal square root can also be written in [[Exponentiation|exponent]] notation, as {{math|''x''<sup>1/2</sup>}}.<ref>{{cite book |title=A First Course in Complex Analysis With Applications |edition=2nd |first1=Dennis G. |last1=Zill |first2=Patrick |last2=Shanahan |publisher=Jones & Bartlett Learning |year=2008 |isbn=0-7637-5772-1 |page=78 |url=https://books.google.com/books?id=YKZqY8PCNo0C |url-status=live |archiveurl=https://web.archive.org/web/20160901081936/https://books.google.com/books?id=YKZqY8PCNo0C |archivedate=2016-09-01 }} [https://books.google.com/books?id=YKZqY8PCNo0C&pg=PA78 Extract of page 78] {{webarchive|url=https://web.archive.org/web/20160901091148/https://books.google.com/books?id=YKZqY8PCNo0C&pg=PA78 |date=2016-09-01 }}</ref>

Square roots of negative numbers can be discussed within the framework of [[complex number]]s. More generally, square roots can be considered in any context in which a notion of "squaring" of some mathematical objects is defined (including [[Matrix (mathematics)|algebras of matrices]], [[endomorphism ring]]s, etc.)

==History==
The [[Yale Babylonian Collection]] [[YBC&nbsp;7289]] clay tablet was created between 1800&nbsp;BC and 1600&nbsp;BC, showing {{sqrt|2}} and {{sqrt|2}}/2 = 1/{{sqrt|2}}  as 1;24,51,10 and 0;42,25,35 [[Sexagesimal |base 60]] numbers on a square crossed by two diagonals.<ref>{{cite web|url=http://www.math.ubc.ca/~cass/Euclid/ybc/analysis.html|title=Analysis of YBC 7289|work=ubc.ca|accessdate=19 January 2015}}</ref> (1;24,51,10) base 60 corresponds to 1.41421296 which is a correct value to 5 decimal points (1.41421356...).

The [[Rhind Mathematical Papyrus]] is a copy from 1650&nbsp;BC of an earlier [[Berlin Papyrus 6619|Berlin Papyrus]] and other texts{{snd}}possibly the [[Kahun Papyrus]]{{snd}}that shows how the Egyptians extracted square roots by an inverse proportion method.<ref>Anglin, W.S. (1994). ''Mathematics: A Concise History and Philosophy''. New York: Springer-Verlag.</ref>

In [[History of India|Ancient India]], the knowledge of theoretical and applied aspects of square and square root was at least as old as the ''[[Sulba Sutras]]'', dated around 800–500&nbsp;BC (possibly much earlier).{{citation needed|date=July 2010|reason=no manuscript dates back that far and reliable secondary sources disagree}} A method for finding very good approximations to the square roots of 2 and 3 are given in the ''[[Baudhayana Sulba Sutra]]''.<ref>Joseph, ch.8.</ref>  [[Aryabhata]] in the ''[[Aryabhatiya]]'' (section 2.4), has given a method for finding the square root of numbers having many digits.

It was known to the ancient Greeks that square roots of [[Natural number|positive whole numbers]] that are not [[Square number|perfect square]]s are always [[irrational number]]s: numbers not expressible as a [[ratio]] of two integers (that is, they cannot be written exactly as ''m/n'', where ''m'' and ''n'' are integers). This is the theorem [[Euclid's Elements|''Euclid X, 9'']] almost certainly due to [[Theaetetus (mathematician)|Theaetetus]] dating back to circa 380&nbsp;BC.<ref>{{cite book
|first= Sir Thomas L. 
|last= Heath
|editor= 
|title= The Thirteen Books of The Elements, Vol. 3
|url=https://archive.org/stream/thirteenbookseu03heibgoog#page/n14/mode/1up
|year=1908
|publisher=Cambridge University Press
|page=3
}}</ref>
The particular case [[Square root of 2|{{sqrt|2}}]] is assumed to date back earlier to the [[Pythagoreanism|Pythagoreans]] and is traditionally attributed to [[Hippasus]].{{Citation needed|date=October 2012}} It is exactly the length of the [[diagonal]] of a [[unit square|square with side length 1]].

In the Chinese mathematical work ''[[Suàn shù shū|Writings on Reckoning]]'', written between 202&nbsp;BC and 186&nbsp;BC during the early [[Han Dynasty]], the square root is approximated by using an "excess and deficiency" method, which says to "...combine the excess and deficiency as the divisor; (taking) the deficiency numerator multiplied by the excess denominator and the excess numerator times the deficiency denominator, combine them as the dividend."<ref>Dauben (2007), p. 210.</ref>

A symbol for square roots, written as an elaborate R, was invented by [[Regiomontanus]] (1436–1476). An R was also used for Radix to indicate square roots in [[Gerolamo Cardano]]'s ''[[Ars Magna (Gerolamo Cardano)|Ars Magna]]''.<ref>{{cite web|url=http://nrich.maths.org/6546|title=The Development of Algebra - 2|work=maths.org|accessdate=19 January 2015|url-status=live|archiveurl=https://web.archive.org/web/20141124102946/http://nrich.maths.org/6546|archivedate=24 November 2014}}</ref>

According to historian of mathematics [[David Eugene Smith|D.E. Smith]], Aryabhata's method for finding the square root was first introduced in Europe by [[Pietro di Giacomo Cataneo|Cataneo]] in 1546.

According to Jeffrey A. Oaks, Arabs used the letter ''[[Gimel#Arabic ĝīm|jīm/ĝīm]]'' ({{lang|ar|ج}}), the first letter of the word “{{lang|ar|جذر}}” (variously transliterated as ''jaḏr'', ''jiḏr'', ''ǧaḏr'' or ''ǧiḏr'', “root”), placed in its initial form ({{lang|ar|ﺟ}}) over a number to indicate its square root. The letter ''jīm'' resembles the present square root shape. Its usage goes as far as the end of the twelfth century  in the works of the Moroccan mathematician [[Ibn al-Yasamin]].<ref>* {{cite thesis | title=Algebraic Symbolism in Medieval Arabic Algebra | first1=Jeffrey A. | last1=Oaks | publisher=Philosophica | year=2012 | page=36 | url=http://logica.ugent.be/philosophica/fulltexts/87-2.pdf | url-status=live | archiveurl=https://web.archive.org/web/20161203134229/http://logica.ugent.be/philosophica/fulltexts/87-2.pdf | archivedate=2016-12-03 }}</ref>

The symbol '√' for the square root was first used in print in 1525 in [[Christoph Rudolff]]'s ''Coss''.<ref>{{Cite book| last=Manguel|first=Alberto| chapter=Done on paper: the dual nature of numbers and the page | title=The Life of Numbers | year=2006 | isbn=84-86882-14-1}}</ref>

==Properties and uses==
[[Image:Square root 0 25.svg|thumb|400px|The graph of the function ''f''(''x'') = {{sqrt|''x''}}, made up of half a [[parabola]] with a vertical [[Directrix (conic section)#Eccentricity, focus and directrix|directrix]] ]]
The principal square root function ''f''(''x'') = {{sqrt|''x''}} (usually just referred to as the "square root function") is a [[function (mathematics)|function]] that maps the [[Set (mathematics)|set]] of nonnegative real numbers onto itself. In [[geometry|geometrical]] terms, the square root function maps the [[area]] of a square to its side length.

The square root of ''x'' is rational if and only if ''x'' is a [[rational number]] that can be represented as a ratio of two perfect squares. (See [[square root of 2]] for proofs that this is an irrational number, and [[quadratic irrational]] for a proof for all non-square natural numbers.) The square root function maps rational numbers into [[algebraic number]]s (a [[superset]] of the rational numbers).

For all real numbers ''x'',

:<math>
\sqrt{x^2} = \left|x\right| = 
\begin{cases} 
  x,  & \mbox{if }x \ge 0 \\
  -x, & \mbox{if }x < 0. 
\end{cases}
</math> &nbsp;&nbsp;&nbsp;&nbsp;(see [[absolute value]])

For all nonnegative real numbers ''x'' and ''y'',

:<math>\sqrt{xy} = \sqrt x \sqrt y</math>

and

:<math>\sqrt x = x^{1/2}.</math>

The square root function is [[Continuous function|continuous]] for all nonnegative ''x'' and [[derivative|differentiable]] for all positive ''x''. If ''f'' denotes the square root function, its derivative is given by:
:<math>f'(x) = \frac{1}{2\sqrt x}.</math>

The [[Taylor series]] of {{sqrt|1 + ''x''}} about ''x'' = 0 converges for {{abs|''x''}} ≤ 1 and is given by

:<math>\sqrt{1 + x} = \sum_{n=0}^\infty \frac{(-1)^n(2n)!}{(1-2n)(n!)^2(4^n)}x^n = 1 + \frac{1}{2}x - \frac{1}{8}x^2 + \frac{1}{16} x^3 - \frac{5}{128} x^4 + \cdots,</math>

The square root of a nonnegative number is used in the definition of [[Euclidean norm]] (and [[Euclidean distance|distance]]), as well as in generalizations such as [[Hilbert space]]s. It defines an important concept of [[standard deviation]] used in probability theory and statistics. It has a major use in the formula for roots of a [[quadratic equation]]; [[quadratic field]]s and rings of [[quadratic integer]]s, which are based on square roots, are important in algebra and have uses in geometry. Square roots frequently appear in mathematical formulas elsewhere, as well as in many [[physics|physical]] laws.

==Square roots of positive integers==
As a positive number has two square roots, one positive, and one negative, which are [[opposite (mathematics)|opposite]] one to the other. So, when talking of ''the square root'' of a positive integer, this is the positive square root that is meant. 

The square roots of an integer are [[algebraic integer]]s and, more specifically, [[quadratic integer]]s.

The square root of a positive integer is the product of the roots of its [[prime number|prime]] factors, because the square root of a product is the product of the square roots of the factors. Since {{math|1={{sqrt|''p''<sup>2''k''</sup>}} = ''p''<sup>''k''</sup>}}, only roots of those primes having an odd power in the [[integer factorization|factorization]] are necessary. More precisely, the square root of a prime factorization is 
:<math>\sqrt{p_1^{2e_1+1}\cdots p_k^{2e_k+1}p_{k+1}^{2e_{k+1}}\dots p_n^{2e_n}}=p_1^{e_1}\dots p_n^{e_n}\sqrt{p_1\dots p_k}.</math> 
 
===As decimal expansions===
The square roots of the [[square number|perfect square]]s (0, 1, 4, 9, 16, etc.) are [[integers]]. In all other cases, the square roots of positive integers are [[irrational number]]s, and therefore their [[decimal representation]]s are non-[[repeating decimal]]s.  Decimal approximations of the square roots of the first few natural numbers are given in the following table.

:{|class="wikitable"
! {{mvar|n}} !! {{math|{{sqrt|''n''}}}}, truncated to 50 decimal places
|- 
|align="right" | 0 || 0
|- 
|align="right" | 1 || 1
|-
|align="right" | 2 || [[Square root of 2|{{gaps|1.4142135623|7309504880|1688724209|6980785696|7187537694}}]]
|-
|align="right" | 3 || [[Square root of 3|{{gaps|1.7320508075|6887729352|7446341505|8723669428|0525381038}}]]
|-
|align="right" | 4 || 2
|-
|align="right" | 5 || [[Square root of 5|{{gaps|2.2360679774|9978969640|9173668731|2762354406|1835961152}}]]
|-
|align="right" | 6 || {{gaps|2.4494897427|8317809819|7284074705|8913919659|4748065667}}
|-
|align="right" | 7 || {{gaps|2.6457513110|6459059050|1615753639|2604257102|5918308245}}
|-
|align="right" | 8 || {{gaps|2.8284271247|4619009760|3377448419|3961571393|4375075389}}
|-
|align="right" | 9 || 3
|-
|align="right" | 10 || {{gaps|3.1622776601|6837933199|8893544432|7185337195|5513932521}}
|-
|}

===As expansions in other numeral systems===
The square roots of the [[square number|perfect square]]s (1, 4, 9, 16, etc.) are integers. In all other cases, the square roots of positive integers are [[irrational number]]s, and therefore their representations in any standard [[positional notation]] system are non-repeating.

The square roots of small integers are used in both the [[SHA-1]] and [[SHA-2]] hash function designs to provide [[nothing up my sleeve number]]s.

===As periodic continued fractions===
One of the most intriguing results from the study of [[irrational number]]s as [[continued fraction]]s was obtained by [[Joseph Louis Lagrange]] {{circa}} 1780. Lagrange found that the representation of the square root of any non-square positive integer as a continued fraction is [[Periodic continued fraction|periodic]]. That is, a certain pattern of partial denominators repeats indefinitely in the continued fraction. In a sense these square roots are the very simplest irrational numbers, because they can be represented with a simple repeating pattern of integers.

:{|
|- 
|align="right"|{{sqrt|2}}|| = [1; 2, 2, ...]
|-
|align="right"|{{sqrt|3}}|| = [1; 1, 2, 1, 2, ...]
|-
|align="right"|{{sqrt|4}}|| = [2]
|-
|align="right"|{{sqrt|5}}|| = [2; 4, 4, ...]
|-
|align="right"|{{sqrt|6}}|| = [2; 2, 4, 2, 4, ...]
|-
|align="right"|{{sqrt|7}}|| = [2; 1, 1, 1, 4, 1, 1, 1, 4, ...]
|-
|align="right"|{{sqrt|8}}||= [2; 1, 4, 1, 4, ...]
|-
|align="right"|{{sqrt|9}}|| = [3]
|-
|align="right"|{{sqrt|10}}|| = [3; 6, 6, ...]
|- 
|align="right"|{{sqrt|11}}|| = [3; 3, 6, 3, 6, ...]
|-
|align="right"|{{sqrt|12}}|| = [3; 2, 6, 2, 6, ...]
|-
|align="right"|{{sqrt|13}}|| = [3; 1, 1, 1, 1, 6, 1, 1, 1, 1, 6, ...]
|-
|align="right"|{{sqrt|14}}|| = [3; 1, 2, 1, 6, 1, 2, 1, 6, ...]
|-
|align="right"|{{sqrt|15}}|| = [3; 1, 6, 1, 6, ...]
|-
|align="right"|{{sqrt|16}}|| = [4]
|-
|align="right"|{{sqrt|17}}|| = [4; 8, 8, ...]
|-
|align="right"|{{sqrt|18}}|| = [4; 4, 8, 4, 8, ...]
|-
|align="right"|{{sqrt|19}}|| = [4; 2, 1, 3, 1, 2, 8, 2, 1, 3, 1, 2, 8, ...]
|-
|align="right"|{{sqrt|20}}|| = [4; 2, 8, 2, 8, ...]
|}

The [[square bracket]] notation used above is a sort of mathematical shorthand to conserve space. Written in more traditional notation the simple continued fraction for the square root of 11, [3; 3, 6, 3, 6, ...], looks like this:

:<math>
\sqrt{11} = 3 + \cfrac{1}{3 + \cfrac{1}{6 + \cfrac{1}{3 + \cfrac{1}{6 + \cfrac{1}{3 + \ddots}}}}}
</math>

where the two-digit pattern {3, 6} repeats over and over again in the partial denominators. Since {{nowrap|1=11 = 3<sup>2</sup> + 2}}, the above is also identical to the following [[generalized continued fraction#Roots of positive numbers|generalized continued fractions]]:

:<math>
\sqrt{11} = 3 + \cfrac{2}{6 + \cfrac{2}{6 + \cfrac{2}{6 + \cfrac{2}{6 + \cfrac{2}{6 + \ddots}}}}} = 3 + \cfrac{6}{20 - 1 - \cfrac{1}{20 - \cfrac{1}{20 - \cfrac{1}{20 - \cfrac{1}{20 - \ddots}}}}}.
</math>

==Computation==
{{Main article|Methods of computing square roots}}
Square roots of positive numbers are not in general [[rational number]]s, and so cannot be written as a terminating or recurring decimal expression. Therefore in general any attempt to compute a square root expressed in decimal form can only yield an approximation, though a sequence of increasingly accurate approximations can be obtained.

Most [[pocket calculator]]s have a square root key. Computer [[spreadsheet]]s and other [[software]] are also frequently used to calculate square roots. Pocket calculators typically implement efficient routines, such as the [[Newton's method]] (frequently with an initial guess of 1), to compute the square root of a positive real number.<ref>{{cite book|last=Parkhurst|first=David F.|title=Introduction to Applied Mathematics for Environmental Science|year=2006|publisher=Springer|isbn=9780387342283|pages=241}}</ref><ref>{{cite book|last=Solow|first=Anita E.|title=Learning by Discovery: A Lab Manual for Calculus|year=1993|publisher=Cambridge University Press|isbn=9780883850831|pages=48}}</ref>  When computing square roots with [[Common logarithm|logarithm table]]s or [[slide rule]]s, one can exploit the identities

:<math>\sqrt{a} = e^{(\ln a)/2} = 10^{(\log_{10} a)/2},</math>

where {{math|ln}} and {{math|log}}<sub>10</sub> are the [[Natural logarithm|natural]] and [[base-10 logarithm]]s.

By trial-and-error,<ref>{{cite book |title=Mathematics for Biological Scientists |first1=Mike |last1=Aitken |first2=Bill |last2=Broadhurst |first3=Stephen |last3=Hladky |publisher=Garland Science |year=2009 |isbn=978-1-136-84393-8 |page=41 |url=https://books.google.com/books?id=KywWBAAAQBAJ |url-status=live |archiveurl=https://web.archive.org/web/20170301101038/https://books.google.com/books?id=KywWBAAAQBAJ |archivedate=2017-03-01 }} [https://books.google.com/books?id=KywWBAAAQBAJ&pg=PA41 Extract of page 41] {{webarchive|url=https://web.archive.org/web/20170301100516/https://books.google.com/books?id=KywWBAAAQBAJ&pg=PA41 |date=2017-03-01 }}</ref> one can square an estimate for {{sqrt|''a''}} and raise or lower the estimate until it agrees to sufficient accuracy. For this technique it is prudent to use the identity

:<math>(x + c)^2 = x^2 + 2xc + c^2,</math>

as it allows one to adjust the estimate ''x'' by some amount ''c'' and measure the square of the adjustment in terms of the original estimate and its square. Furthermore, (''x'' + ''c'')<sup>2</sup> ≈ ''x''<sup>2</sup> + 2''xc'' when ''c'' is close to 0, because the [[tangent line]] to the graph of ''x''<sup>2</sup> + 2''xc'' + ''c''<sup>2</sup> at ''c'' = 0, as a function of ''c'' alone, is ''y'' = 2''xc'' + ''x''<sup>2</sup>. Thus, small adjustments to ''x'' can be planned out by setting 2''xc'' to ''a'', or ''c'' = ''a''/(2''x'').

The most common [[iterative method]] of square root calculation by hand is known as the "[[Babylonian method]]" or "Heron's method" after the first-century Greek philosopher [[Hero of Alexandria|Heron of Alexandria]], who first described it.<ref>{{cite book
  | last = Heath
  | first = Sir Thomas L.
  | authorlink = 
  | coauthors = 
  | title = A History of Greek Mathematics, Vol. 2
  | publisher = Clarendon Press
  | year = 1921
  | location = Oxford
  | pages = 323–324
  | url = https://books.google.com/?id=LOA5AAAAMAAJ&pg=PR323
  | doi = 
  | id = 
  | isbn = }}</ref>
The method uses the same iterative scheme as the [[Newton–Raphson method]] yields when applied to the function y = ''f''(''x'') = ''x''<sup>2</sup> − ''a'', using the fact that its slope at any point is ''dy''/''dx'' = ''{{prime|f}}''(''x'') = 2''x'', but predates it by many centuries.<ref>{{Cite book
 |title      = Elementary functions: algorithms and implementation
 |first1     = Jean-Mic
 |last1      = Muller
 |publisher  = Springer
 |year       = 2006
 |isbn       = 0-8176-4372-9
 |pages      = 92–93
 |url        = https://books.google.com/?id=g3AlWip4R38C
 |postscript = <!--None-->
}}, [https://books.google.com/books?id=g3AlWip4R38C&pg=PA92 Chapter 5, p 92] {{webarchive|url=https://web.archive.org/web/20160901091516/https://books.google.com/books?id=g3AlWip4R38C&pg=PA92 |date=2016-09-01 }}
</ref>
The algorithm is to repeat a simple calculation that results in a number closer to the actual square root each time it is repeated with its result as the new input. The motivation is that if ''x'' is an overestimate to the square root of a nonnegative real number ''a'' then ''a''/''x''<!-- please avoid intermixing <math> with a bare text in one paragraph: it is the ugliest pair among 3 existing styles --> will be an underestimate and so the average of these two numbers is a better approximation than either of them. However, the [[inequality of arithmetic and geometric means]] shows this average is always an overestimate of the square root (as noted [[Square root#Geometric construction of the square root|below]]), and so it can serve as a new overestimate with which to repeat the process, which [[Limit of a sequence|converges]] as a consequence of the successive overestimates and underestimates being closer to each other after each iteration. To find ''x'':

# Start with an arbitrary positive start value ''x''. The closer to the square root of ''a'', the fewer the iterations that will be needed to achieve the desired precision.
# Replace ''x'' by the average (''x'' + ''a''/''x'') / 2 between ''x'' and ''a''/''x''.
# Repeat from step 2, using this average as the new value of ''x''.

That is, if an arbitrary guess for {{sqrt|''a''}} is ''x''<sub>0</sub>, and {{nowrap|1 = ''x''<sub>''n'' + 1</sub> = (''x<sub>n</sub>'' + ''a''/''x<sub>n</sub>'') / 2}}, then each x<sub>n</sub> is an approximation of {{sqrt|''a''}} which is better for large ''n'' than for small ''n''. If ''a'' is positive, the convergence is [[Rate of convergence|quadratic]], which means that in approaching the limit, the number of correct digits roughly doubles in each next iteration. If {{nowrap|1 =''a'' = 0}}, the convergence is only linear.

Using the identity

:<math>\sqrt{a} = 2^{-n}\sqrt{4^n a},</math>

the computation of the square root of a positive number can be reduced to that of a number in the range {{closed-open|1,4}}. This simplifies finding a start value for the iterative method that is close to the square root, for which a [[Polynomial function|polynomial]] or [[Piecewise linear function|piecewise-linear]] [[Approximation theory|approximation]] can be used.

The [[Computational complexity theory|time complexity]] for computing a square root with ''n'' digits of precision is equivalent to that of multiplying two ''n''-digit numbers.

Another useful method for calculating the square root is the [[shifting nth root algorithm]], applied for {{nowrap|1= ''n'' = 2}}.

The name of the square root [[Function (programming)|function]] varies from [[programming language]] to programming language, with <code>sqrt</code><ref>{{cite web |title=Function sqrt |work=CPlusPlus.com |date=2016 |publisher=The C++ Resources Network |url=http://www.cplusplus.com/reference/clibrary/cmath/sqrt/ |access-date=June 24, 2016 |url-status=live |archiveurl=https://web.archive.org/web/20121122050619/http://www.cplusplus.com/reference/clibrary/cmath/sqrt/ |archivedate=November 22, 2012 }}</ref> (often pronounced "squirt" <ref>{{cite book |title=C++ for the Impatient |first=Brian |last=Overland |page=338 |publisher=Addison-Wesley |date=2013 |isbn=9780133257120 |oclc=850705706 |url=https://books.google.com/books?id=eJFpV-_t4WkC&pg=PA338&dq=%22squirt%22+sqrt+C%2B%2B&hl=en&sa=X&ved=0ahUKEwjEwfj04sHNAhUY0GMKHatGDnsQ6AEIKDAC#v=onepage&q=%22squirt%22%20sqrt%20C%2B%2B&f=false |access-date=June 24, 2016 |url-status=live |archiveurl=https://web.archive.org/web/20160901082021/https://books.google.com/books?id=eJFpV-_t4WkC&pg=PA338&dq=%22squirt%22+sqrt+C%2B%2B&hl=en&sa=X&ved=0ahUKEwjEwfj04sHNAhUY0GMKHatGDnsQ6AEIKDAC#v=onepage&q=%22squirt%22%20sqrt%20C%2B%2B&f=false |archivedate=September 1, 2016 }}</ref>) being common, used in [[C (programming language)|C]], [[C++]], and derived languages like [[JavaScript]], [[PHP]], and [[Python (programming language)|Python]].

==Square roots of negative and complex numbers==<!-- This section is linked from [[Complex number]] -->
{{multiple image |align=left |direction=horizontal 
 |image1=Complex sqrt leaf1.jpg |caption1=First leaf of the complex square root
 |image2=Complex sqrt leaf2.jpg |caption2=Second leaf of the complex square root
 |image3=Riemann surface sqrt.svg |caption3=Using the [[Riemann surface]] of the square root, it is shown how the two leaves fit together
}}
{{clear}}
The square of any positive or negative number is positive, and the square of 0 is 0. Therefore, no negative number can have a [[real number|real]] square root.  However, it is possible to work with a more inclusive set of numbers, called the [[complex number]]s, that does contain solutions to the square root of a negative number.  This is done by introducing a new number, denoted by ''i'' (sometimes ''j'', especially in the context of [[electric current|electricity]] where "''i''" traditionally represents electric current) and called the [[imaginary unit]], which is ''defined'' such that {{nowrap|1=''i''<sup>2</sup> = −1}}. Using this notation, we can think of ''i'' as the square root of −1, but we also have {{nowrap|1=(−''i'')<sup>2</sup> = ''i''<sup>2</sup> = −1}} and so −''i'' is also a square root of −1. By convention, the principal square root of −1 is ''i'', or more generally, if ''x'' is any nonnegative number, then the principal square root of −''x'' is

:<math>\sqrt{-x} = i \sqrt x.</math>

The right side (as well as its negative) is indeed a square root of −''x'', since

:<math>(i\sqrt x)^2 = i^2(\sqrt x)^2 = (-1)x = -x.</math>

For every non-zero complex number ''z'' there exist precisely two numbers ''w'' such that {{nowrap|1=''w''<sup>2</sup> = ''z''}}: the principal square root of ''z'' (defined below), and its negative.

===Principal square root of a complex number===
{{Visualisation complex number roots}}
To find a definition for the square root that allows us to consistently choose a single value, called the [[principal value]], we start by observing that any complex number ''x'' + ''iy'' can be viewed as a point in the plane, (''x'', ''y''), expressed using [[Cartesian coordinate system|Cartesian coordinates]]. The same point may be reinterpreted using [[polar coordinates]] as the pair <math>(r, \varphi</math>), where ''r'' ≥ 0 is the distance of the point from the origin, and  <math>\varphi</math> is the angle that the line from the origin to the point makes with the positive real (''x'') axis. In complex analysis, the location of this point is conventionally written <math>re^{i\varphi}.</math> If

:<math> z=r e^{i \varphi} \text{ with } -\pi < \varphi \le \pi, </math>

then we define the principal square root of ''z'' as follows:

:<math>\sqrt{z} = \sqrt{r} e^{i \varphi / 2}.</math>

The principal square root function is thus defined using the nonpositive real axis as a [[branch cut]].  The principal square root function is [[holomorphic function|holomorphic]] everywhere except on the set of non-positive real numbers (on strictly negative reals it isn't even [[continuous function|continuous]]). The above Taylor series for {{sqrt|1 + ''x''}} remains valid for complex numbers ''x'' with {{nowrap|{{abs|''x''}} < 1}}.

The above can also be expressed in terms of [[trigonometric function]]s:
:<math>\sqrt{r \left(\cos \varphi + i \sin \varphi \right)} = \sqrt{r} \left ( \cos \frac{\varphi}{2} + i \sin \frac{\varphi}{2} \right ) .</math>

===Algebraic formula===
[[Image:Imaginary2Root.svg|right|thumb|The square roots of {{mvar|i}}]]
When the number is expressed using Cartesian coordinates the following formula can be used for the principal square root:<ref>{{cite book
 |title       = Handbook of mathematical functions with formulas, graphs, and mathematical tables
 |first1      = Milton
 |last1       = Abramowitz
 |first2      = Irene A.
 |last2       = Stegun
 |publisher   = Courier Dover Publications
 |year        = 1964
 |isbn        = 0-486-61272-4
 |page        = 17
 |url         = https://books.google.com/books?id=MtU8uP7XMvoC
 |url-status     = live
 |archiveurl  = https://web.archive.org/web/20160423180235/https://books.google.com/books?id=MtU8uP7XMvoC
 |archivedate = 2016-04-23
}}, [http://www.math.sfu.ca/~cbm/aands/page_17.htm Section 3.7.27, p. 17] {{webarchive|url=https://web.archive.org/web/20090910094533/http://www.math.sfu.ca/~cbm/aands/page_17.htm |date=2009-09-10 }}
</ref><ref>{{cite book
 |title       = Classical algebra: its nature, origins, and uses
 |first1      = Roger
 |last1       = Cooke
 |publisher   = John Wiley and Sons
 |year        = 2008
 |isbn        = 0-470-25952-3
 |page        = 59
 |url         = https://books.google.com/books?id=lUcTsYopfhkC&pg=PA59
 |url-status     = live
 |archiveurl  = https://web.archive.org/web/20160423183239/https://books.google.com/books?id=lUcTsYopfhkC&pg=PA59
 |archivedate = 2016-04-23
}}
</ref>
:<math>\sqrt{x+iy} = \sqrt{\frac{\sqrt{x^2+y^2} + x}{2}} \pm i\sqrt{\frac{\sqrt{x^2+y^2} - x}{2}},</math>
where the [[Sign function|sign]] of the imaginary part of the root is taken to be the same as the sign of the imaginary part of the original number, or positive when zero. The real part of the principal value is always nonnegative.

For example, the principal square roots of {{math|&plusmn;''i''}} are given by:
:<math>\begin{align}
   \sqrt{i} &= \frac{1}{\sqrt{2}} + i\frac{1}{\sqrt{2}} = \frac{\sqrt{2}}{2}(1+i),\\
   \sqrt{-i} &= \frac{1}{\sqrt{2}} - i\frac{1}{\sqrt{2}} = \frac{\sqrt{2}}{2}(1-i).
 \end{align}</math>

===Notes===

In the following, the complex ''z'' and ''w'' may be expressed as:

* <math> z=|z|e^{i \theta_z}</math>
* <math> w=|w|e^{i \theta_w}</math>

where <math>-\pi<\theta_z\le\pi</math> and <math>-\pi<\theta_w\le\pi</math>.

Because of the discontinuous nature of the square root function in the complex plane, the following laws are '''not true''' in general.

*  <math>\sqrt{zw} = \sqrt{z} \sqrt{w}</math> (counterexample for the principal square root: {{math|1=''z'' = −1}} and  {{math|1=''w'' = −1}}) This equality is valid only when <math>-\pi<\theta_z+\theta_w\le\pi</math>
* <math>\frac{\sqrt{w}}{\sqrt z} = \sqrt{\frac{w}{z}}</math> (counterexample for the principal square root: {{math|1=''w'' = 1}} and {{math|1=''z'' = −1}})This equality is valid only when <math>-\pi<\theta_w-\theta_z\le\pi</math>
*<math>\sqrt{z^*} = \left( \sqrt z \right)^*</math> (counterexample for the principal square root: {{math|1=''z'' = −1}})This equality is valid only when <math>\theta_z\ne\pi</math>

A similar problem appears with other complex functions with branch cuts, e.g., the [[complex logarithm]] and the relations {{math|1=log&thinsp;''z'' + log&thinsp;''w'' = log(''zw'')}} or {{math|1=log&thinsp;(''z''<sup>*</sup>) = log&thinsp;(''z'')<sup>*</sup>}} which are not true in general.

Wrongly assuming one of these laws underlies several faulty "proofs", for instance the following one showing that {{math|1=−1 = 1}}:

:<math>
\begin{align}
-1 &= i \cdot i \\
&= \sqrt{-1} \cdot \sqrt{-1} \\
&= \sqrt{\left(-1\right)\cdot\left(-1\right)} \\
&= \sqrt{1} \\
&= 1
\end{align}
</math>

The third equality cannot be justified (see [[invalid proof]]). It can be made to hold by changing the meaning of √ so that this no longer represents the principal square root (see above) but selects a branch for the square root that contains ({{sqrt|−1}})·({{sqrt|−1}}). The left-hand side becomes either

:<math>\sqrt{-1} \cdot \sqrt{-1}=i \cdot i=-1</math>

if the branch includes +''i'' or

:<math>\sqrt{-1} \cdot \sqrt{-1}=(-i) \cdot (-i)=-1</math>

if the branch includes −''i'', while the right-hand side becomes

:<math>\sqrt{\left(-1\right)\cdot\left(-1\right)}=\sqrt{1}=-1,</math>

where the last equality, {{math|1={{sqrt|1}} = −1}}, is a consequence of the choice of branch in the redefinition of √.

==Square roots of matrices and operators==
{{Main article|Square root of a matrix}}
{{See also|Square root of a 2 by 2 matrix}}

If ''A'' is a [[positive-definite matrix]] or operator, then there exists precisely one positive definite matrix or operator ''B'' with {{nowrap|1=''B''<sup>2</sup> = ''A''}}; we then define {{nowrap|1=''A''<sup>1/2</sup> = ''B''}}. In general matrices may have multiple square roots or even an infinitude of them.  For example, the {{nowrap|2 × 2}} [[identity matrix]] has an infinity of square roots,<ref>Mitchell, Douglas W., "Using Pythagorean triples to generate square roots of I<sub>2</sub>", ''Mathematical Gazette'' 87, November 2003, 499–500.</ref> though only one of them is positive definite.

==In integral domains, including fields==
Each element of an [[integral domain]] has no more than 2 square roots. The [[difference of two squares]] identity {{math|1=''u''<sup>2</sup> − ''v''<sup>2</sup> = (''u'' − ''v'')(''u'' + ''v'')}} is proved using the [[commutative ring|commutativity of multiplication]]. If {{mvar|u}} and {{mvar|v}} are square roots of the same element, then {{math|1=''u''<sup>2</sup> − ''v''<sup>2</sup> = 0}}. Because there are no [[zero divisors]] this implies {{math|1=''u'' = ''v''}} or {{math|1=''u'' + ''v'' = 0}}, where the latter means that two roots are [[additive inverse]]s of each other. In other words if an element a square root {{mvar|u}} of an element {{mvar|a}} exists, then the only square roots of {{mvar|a}} are {{mvar|u}} and {{mvar|&minus;u}}. The only square root of 0 in an integral domain is 0 itself.

In a field of [[characteristic (algebra)|characteristic]]&nbsp;2, an element either has one square root or does not have any at all, because each element is its own additive inverse, so that {{math|1=&minus;''u'' = ''u''}}. If the field is [[finite field|finite]] of characteristic&nbsp;2 then every element has a unique square root. In a [[field (mathematics)|field]] of any other characteristic, any non-zero element either has two square roots, as explained above, or does not have any.

Given an odd [[prime number]] {{mvar|p}}, let {{math|1=''q'' = ''p''<sup>''e''</sup>}} for some positive integer {{mvar|e}}. A non-zero element of the field [[finite field|{{math|'''F'''<sub>''q''</sub>}}]] with {{mvar|q}} elements is a [[quadratic residue]] if it has a square root in {{math|'''F'''<sub>''q''</sub>}}. Otherwise, it is a quadratic non-residue. There are {{math|(''q'' − 1)/2}} quadratic residues and {{math|(''q'' − 1)/2}} quadratic non-residues; zero is not counted in either class. The quadratic residues form a [[group (mathematics)|group]] under multiplication. The properties of quadratic residues are widely used in [[number theory]].

==In rings in general==
Unlike in an integral domain, a square root in an arbitrary (unital) ring need not be unique up to sign.  For example, in the ring <math>\mathbb{Z}/8\mathbb{Z}</math> of integers [[modular arithmetic|modulo 8]] (which is commutative, but has zero divisors), the element 1 has four distinct square roots: ±1 and ±3.

Another example is provided by the ring of [[quaternion]]s <math>\mathbb{H},</math> which has no zero divisors, but is not commutative.  Here, the element −1 has [[quaternion#Square roots of −1|infinitely many square roots]], including {{math|±''i''}}, {{math|±''j''}}, and {{math|±''k''}}.  In fact, the set of square roots of −1 is exactly

:<math>\{ai + bj + ck \mid a^2 + b^2 + c^2 = 1\} .</math>

A square root of 0 is either 0 or a zero divisor.  Thus in rings where zero divisors do not exist, it is uniquely 0.  However, rings with zero divisors may have multiple square roots of 0.  For example, in <math>\mathbb{Z}/n^2\mathbb{Z},</math> any multiple of {{mvar|n}} is a square root of 0.

==Geometric construction of the square root==
[[File:Euclid Corollary 5.svg|thumb|The [[Spiral of Theodorus]] up to the triangle with a hypotenuse of {{math|{{radic|4}}}} ({{math|1 {{=}} {{sqrt|1}}}})]]

The square root of a positive number is usually defined as the side length of a [[square]] with the [[area]] equal to the given number. But the square shape is not necessary for it: if one of two [[similarity (geometry)|similar]] [[Euclidean plane|planar Euclidean]] objects has the area ''a'' times greater than another, then the ratio of their linear sizes is {{sqrt|''a''}}.

A square root can be constructed with a compass and straightedge. In his [[Euclid's Elements|Elements]], [[Euclid]] ([[floruit|fl.]] 300&nbsp;BC) gave the construction of the [[geometric mean]] of two quantities in two different places: [http://aleph0.clarku.edu/~djoyce/java/elements/bookII/propII14.html Proposition II.14] and [http://aleph0.clarku.edu/~djoyce/java/elements/bookVI/propVI13.html Proposition VI.13].  Since the geometric mean of ''a'' and ''b'' is {{sqrt|''ab''}}, one can construct {{sqrt|''a''}} simply by taking {{nowrap|1=''b'' = 1}}.

The construction is also given by [[Descartes]] in his ''[[La Géométrie]]'', see figure 2 on [http://historical.library.cornell.edu/cgi-bin/cul.math/docviewer?did=00570001&seq=12&frames=0&view=50 page 2]. However, Descartes made no claim to originality and his audience would have been quite familiar with Euclid.

Euclid's second proof in Book VI depends on the theory of [[Similar triangles#Similar triangles|similar triangles]]. Let AHB be a line segment of length {{nowrap|''a'' + ''b''}} with {{nowrap|1=AH = ''a''}} and {{nowrap|1=HB = ''b''}}. Construct the circle with AB as diameter and let C be one of the two intersections of the perpendicular chord at H with the circle and denote the length CH as ''h''. Then, using [[Thales' theorem]] and, as in the [[Pythagorean theorem#Proof using similar triangles|proof of Pythagoras' theorem by similar triangles]], triangle AHC is similar to triangle CHB (as indeed both are to triangle ACB, though we don't need that, but it is the essence of the proof of Pythagoras' theorem) so that AH:CH is as HC:HB, i.e. {{nowrap|1= ''a''/''h'' = ''h''/''b''}}, from which we conclude by cross-multiplication that {{nowrap|1= ''h''<sup>2</sup> = ''ab''}}, and finally that {{nowrap|1=''h'' = {{sqrt|''ab''}}}}. When marking the midpoint O of the line segment AB and drawing the radius OC of length {{nowrap|(''a'' + ''b'')/2}}, then clearly OC > CH, i.e. {{nowrap|(''a'' + ''b'')/2 ≥ {{sqrt|''ab''}}}} (with equality if and only if {{nowrap|1=''a'' = ''b''}}), which is the [[inequality of arithmetic and geometric means|arithmetic–geometric mean inequality for two variables]] and, as noted [[Square root#Computation|above]], is the basis of the [[Greek Mathematics|Ancient Greek]] understanding of "Heron's method".

Another method of geometric construction uses [[right triangle]]s and [[Mathematical induction|induction]]: {{sqrt|1}} can, of course, be constructed, and once {{sqrt|''x''}} has been constructed, the right triangle with legs 1 and {{sqrt|''x''}} has a [[hypotenuse]] of {{sqrt|''x'' + 1}}. Constructing successive square roots in this manner yields the [[Spiral of Theodorus]] depicted above.

==See also==
* [[Apotome (mathematics)]]
* [[Cube root]]
* [[Integer square root]]
* [[Nested radical]]
* [[Nth root]]
* [[Root of unity]]
* [[Solving quadratic equations with continued fractions]]
* [[Square root principle]]
* [[Quantum gate#Square root of NOT gate (√NOT)|The square root of NOT gate ({{sqrt|NOT}})]], one of the [[logic gate]]s used in [[quantum computer]]s (doesn't exist for non-quantum where [[NOT gate]]s are used)

==Notes==
{{Reflist}}

==References==
* {{cite book | last = Dauben | first = Joseph W. | author-link = Joseph Dauben | editor-last = Katz | editor-first = Victor J. | title = The Mathematics of Egypt, Mesopotamia, China, India, and Islam | chapter = Chinese Mathematics I | publisher = Princeton University Press | location = Princeton | year = 2007 | url = https://books.google.com/books?id=3ullzl036UEC | isbn = 0-691-11485-4}}
* {{cite book|title=Algebra|edition=3rd|first1=Izrael M.|last1=Gel'fand|first2=Alexander|author1-link=Israel Gelfand|last2=Shen|publisher=Birkhäuser|year=1993|isbn=0-8176-3677-3|page=120|url=https://books.google.com/books?id=Z9z7iliyFD0C}}
* {{cite book | last = Joseph | first = George | title = The Crest of the Peacock | publisher = Princeton University Press | location = Princeton | year = 2000 | isbn = 0-691-00659-8 | url = https://archive.org/details/crestofpeacockno00jose }}
* {{cite book | last = Smith | first = David | author-link = David Eugene Smith| title = History of Mathematics | publisher = Dover Publications | location = New York | year = 1958 | isbn = 978-0-486-20430-7 | volume = 2}}
* {{citation|last=Selin|first=Helaine|author-link=Helaine Selin|title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures|url=https://books.google.com/books?id=kt9DIY1g9HYC&pg=PA1268|year=2008|publisher=Springer|isbn=978-1-4020-4559-2}}.

==External links==
{{commons category}}
* [http://www.azillionmonkeys.com/qed/sqroot.html Algorithms, implementations, and more]{{snd}}Paul Hsieh's square roots webpage
* [http://johnkerl.org/doc/square-root.html How to manually find a square root]
* [http://www.ams.org/samplings/feature-column/fc-2013-05 AMS Featured Column, Galileo's Arithmetic by Tony Philips]{{snd}}includes a section on how Galileo found square roots

{{DEFAULTSORT:Square Root}}
[[Category:Elementary special functions]]
[[Category:Elementary mathematics]]
[[Category:Unary operations]]