Balian–Low theorem

In mathematics, the Balian–Low theorem in Fourier analysis is named for Roger Balian and Francis E. Low. The theorem states that there is no well-localized window function (or Gabor atom) g either in time or frequency for an exact Gabor frame (Riesz Basis).

Statement

Suppose g is a square-integrable function on the real line, and consider the so-called Gabor system

$g_{m,n}(x)=e^{2\pi imbx}g(x-na),$

for integers m and n, and a,b>0 satisfying ab=1. The Balian–Low theorem states that if

$\{g_{m,n}:m,n\in \mathbb {Z} \}$

is an orthonormal basis for the Hilbert space

$L^{2}(\mathbb {R} ),$

then either

$\int _{-\infty }^{\infty }x^{2}|g(x)|^{2}\;dx=\infty \quad {\textrm {or}}\quad \int _{-\infty }^{\infty }\xi ^{2}|{\hat {g}}(\xi )|^{2}\;d\xi =\infty .$

Generalizations

The Balian–Low theorem has been extended to exact Gabor frames.

See also

• Gabor filter (in image processing)

References

• Template:Cite journal