Transcription of Fourier transform techniques 1 The Fourier transform
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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. Settingkn=n /Land k= /Land using (1), onecan writef(x) =12 n= ( L Lf(y)e iknydy)eiknx this is a Riemann sum for an integral on the intervalk ( , ).
The function F(k) is the Fourier transform of f(x). The inverse transform of F(k) is given by the formula (2). (Note that there are other conventions used to define the Fourier transform). Instead of capital letters, we often use the notation f^(k) for the Fourier transform, and F (x) for the inverse transform. 1.1 Practical use of the Fourier ...
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