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Fourier transform techniques 1 The Fourier transform

Fourier transform techniques1 The Fourier transformRecall for a functionf(x) : [ L,L] C, we have the orthogonal expansionf(x) = n= cnein x/L, cn=12L L Lf(y)e in y/Ldy.(1)We think ofcnas representing the amount of a particular eigenfunction with wavenumberkn=n /Lpresent in the functionf(x). So what ifLgoes to ? Notice that the allowedwavenumbers become more and more dense. Therefore, whenL= , we expectf(x)is a su-perposition of an uncountable number of waves corresponding to every wavenumberk R,which can be accomplished by writingf(x)as a integral overkinstead of a sum let s take the limitL formally.

This is very troublesome: the integral does not even converge, so what could such a statement mean? Of course the issue is that the integral represents a distribution, not a regular function. We can define the inverse transform of F(k) more generally as a distribution which is the limit of the regular functions f L(x) = 1 2ˇ Z L L exp(ikx)F(k)dk

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