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Fourier Series to Fourier Integral

  1. Jun 16, 2012 #1
    1. Question

    [tex] Consider\ any\ periodic\ function\ f(x)\ of\ period\ 2L\ that\ can\ be\ represented\ by\ a\ Fourier\ series:\

    {f(x)= a_0 + \sum_{n=1}^\infty\ a_ncos\ w_nx + b_nsin\ w_nx}\ ,\ w_n= {n\pi x\over \ L } [/tex]



    [tex] How\ do\ I\ get\ this\ form\ :\ f(x)= \int_{0}^{\infty}\ [A(w)cos\ wx + B(w)sin\ wx ]\ dw\ ,\ A(w)= {1\over \pi}\int_{-\infty}^{\infty}\ f(v)cos\ wv\ dv\ , B(w) = {1\over \pi}\int_{-\infty}^{\infty}\ f(v)sin\ wv\ dv? [/tex]



    2. The attempt at a solution

    [tex] Denoting\ the\ variable\ of\ integration\ by\ v(why\ so?)\ i.e.\ f(x)= {1\over 2L}\int_{-L}^{L}\ {f(v)}\ dv + {1\over L}\sum_{n=1}^\infty\ [(cos\ w_nx) \int_{-L}^{L}\ {f(v)cos\ w_nv}\ dv + (sin\ w_nx) \int_{-L}^{L}\ {f(v)sin\ w_nv}\ dv][/tex]


    [tex] To\ convert\ to\ a\ Fourier\ integral,\ set\ \Delta w= w_{n+1} - w_n = {\pi\over \ L}\ ,it\ follows\ that\ f(x)= {1\over 2L}\int_{-L}^{L}\ {f(v)}\ dv + {1\over \pi}\sum_{n=1}^\infty\ [(cos\ w_nx) \Delta w\int_{-L}^{L}\ {f(v)cos\ w_nv}\ dv + (sin\ w_nx) \Delta w\int_{-L}^{L}\ {f(v)sin\ w_nv}\ dv] [/tex]
     
    Last edited: Jun 16, 2012
  2. jcsd
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