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Problem with symbolic Fourier Transform

Asked by Halil Eyyuboglu on 17 Jul 2019
Latest activity Answered by Halil Eyyuboglu on 21 Jul 2019
Hello, I runing the following code, the answer given for the unit step function expressed as the difference of two heaviside functions seems to be in agreement with the ones found in the literature, but the arguments of delta functions given as answer the to the Fourier Transform of the cosine contain (the unnecessary) 2pi also the amplitude seems to be multiplied by 2pi. To me, this is not in agreement with the literature.
Awaiting your kind comments
syms f_0 t f T A
sympref('FourierParameters',[1 -2*sym(pi)]);
F = A*cos(2*pi*f_0*t);
F_FT = fourier(F,t,f)
F = A*(heaviside(t) - heaviside(t-T));
F_FT = fourier(F,t,f)

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2 Answers

Answer by David Goodmanson on 18 Jul 2019
Edited by David Goodmanson on 18 Jul 2019

Hello Halil,
For the delta function,
delta(x-x0) = |a| delta(a(x-x0))
so with a = 2*pi, both versions are correct.
quick look, assuming a>0:
Int delta(x-x0) dx = 1
let x = a y, x0 = a y0, and dx = a dy (which is key) then
Int delta(a(y-y0)) a dy = 1
then replace y by x
Int a delta(a(x-x0)) dx = 1
this function and delta(x-x0) have identical behavior and are equal.
For a<0 there is a bit of messing around with reversing the limits of integration, using the fact that delta is an even function of its argument and so on. You end up with |a| in front.

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Answer by Halil Eyyuboglu on 21 Jul 2019

Thanks for the comment, it is now verified.

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