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Homework Help: Fourier transform convolution proof

  1. May 5, 2014 #1
    1. The problem statement, all variables and given/known data

    Let FT(f) = Fourier transform of f, (f*g)(x) = convolution of f and g.
    Given FT(f*g) = FT(f)FT(g), the first part of the convolution theorem, show that FT[fg] = [FT(f)*FT(g)]/2pi.

    2. Relevant equations

    Duality: FT2f(x) = (2pi)f(-x)

    Convolution: (f*g)(x) = ∫f(u)g(x-u)du

    3. The attempt at a solution

    FT(f*g) = FT(f)FT(g)
    FT2(f*g) = FT[FT(f)FT(g)]
    LHS = FT2(f*g)(x) = (2pi)(f*g)(-x) (by duality) =(2pi)∫f(u)g(-x-u)du (by convolution) = (2pi)∫{[FT2f(-u)][FT2g(x+u)]}/(2pi)2 du
    implying that RHS = FT[FT(f)FT(g)] = (1/2pi)∫FT[FT(f(-u))]FT[FT(g(x+u))] du ≠ FT[fg] = [FT(f)*FT(g)]/2pi

    Where did I go wrong?

  2. jcsd
  3. May 5, 2014 #2


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    Try letting f = FT(F) and g=FT(G). Then evaluate FT2(f*g) and FT[FT(f)FT(g)] separately and show they are equal.
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