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Paley-wiener Theorem




Generally, the Fourier transform can be defined for any Tempered Distribution ; moreover, any distribution of compact support ''v'' is a tempered distribution. If ''v'' is a distribution of compact support and ''f'' is an infinitely differentiable function, the expression

: v(f) = v_x \left(f(x) ight)

is well defined. In the above expression the variable ''x'' in ''vx'' is a dummy variable and indicates that the distribution is to be applied with the argument function considered as a function of ''x''.

It can be shown that the Fourier transform of ''v'' is a function (as opposed to a general tempered distribution) given at the value ''s'' by

: \hat{v}(s) = (2 \pi)^{-n/2} v_x\left(e^{-i \langle x, s angle} ight)

and that this function can be extended to values of ''s'' in the complex space Cn. This extension of the Fourier transform to the complex domain is called the Fourier-Laplace Transform .

Theorem. An entire function ''F'' on '''C'''n is the Fourier-Laplace transform of distribution ''v'' of compact support if and only if for all ''z'' ∈ '''C'''''n'',