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Homework Help: Fourier Coefficients Property

  1. Jul 14, 2010 #1
    1. The problem statement, all variables and given/known data

    Let f be a C1 function on [-pi,pi]. Prove the Fourier coefficients of f satisfy

    |an| <= K/n and |bn| <= L/n n=1,2,...

    2. Relevant equations

    an = 1/pi * int[-pi..pi] (f(x)*cos(nx)) dx

    bn = 1/pi * int[-pi..pi] (f(x)*sin(nx)) dx

    Sorry if my form is slightly unpleasing to the eye, but I'm sure if you're reading my post you probably know what I'm talking about.

    3. The attempt at a solution

    |an| = | 1/pi * int[-pi..pi] (f(x)*cos(nx)) dx |

    <= 1/pi * int[-pi..pi] | (f(x)*cos(nx)) | dx

    and that's as far as I could get. I thought maybe I could show that

    int[-pi..pi]( | cosnx | )dx <= (1/n)*int[-pi..pi]( |cosx| ) dx

    but that turned out to be false.

    Any ideas?
     
  2. jcsd
  3. Jul 15, 2010 #2

    lanedance

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    not there, but some ideas that are hopefully helpful
    so I = [itex] \left[ -\pi, \pi \right] [/itex] is a closed interval

    as the function is [itex] C^1 [/itex] both f & f' are continuous on I and so attain a maximum on I

    [tex]f = a_0 + \sum_{n=1} (a_n cos(nx) + b_n sin(nx)) [/tex]

    [tex]f' = \sum_{n=1} (-n.a_n sin(nx) + n .b_n cos(nx)) [/tex]

    as f and f' are bounded, then applying Parseval's theorem, for some positive number M
    [tex]\int f'^2 = M [/tex]
    [tex]= \pi \left( \sum_{n=1} (n.a_n)^2 + (n .b_n)^2 \right) [/tex]
     
  4. Jul 15, 2010 #3
    Ok I followed everything you said but I still don't see how this relates to what I'm trying to show. The fact that there is a /n necessary is really confusing me.

    i.e. |an| <= K/n and not just K


    Edit:

    Ok I may have gotten it.

    I think the solution is |an| <= |a1|/n and |bn| <= |b1|/n

    There's too much to write out how I got this but does this seem correct?
     
    Last edited: Jul 15, 2010
  5. Jul 15, 2010 #4

    vela

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    The 1/n factor is the reason lanedance looked at the series for f' rather than f. Look at the series from Parseval's identity. What can you say about the sign of each term?
    No, this isn't correct. f(x)=sin(2x) is a simple counterexample since it has a1=0 and a2=1.
     
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