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Homework Help: Need help solving what seems like a strange integral

  1. Nov 21, 2015 #1
    1. The problem statement, all variables and given/known data
    I need to find the Fourier series for [itex]f(x)=|x|^3[/itex].

    2. Relevant equations
    [itex]A_n=\frac{2}{L}\int_{0}^{L}|x|^3cos{\frac{n\pi x}{L}}dx[/itex]
    [itex]\int_{0}^{L}x|x|sin{\frac{n\pi x}{L}}dx[/itex]

    3. The attempt at a solution
    Since it is an even function I know that it will be a cosine series and so I set out to find the A coefficient like so:
    [itex]A_n=\frac{2}{L}\int_{0}^{L}|x|^3cos{\frac{n\pi x}{L}}dx[/itex]

    Through integration by parts I end up with a sine term that goes to zero and some coefficients out in front of an integral that looks like:
    [itex]\int_{0}^{L}x|x|sin{\frac{n\pi x}{L}}dx[/itex]

    I would really appreciate any help with this particular integral or if someone could point me in the direction of a solution where a Fourier series is calculated for a function with an absolute value to the power>2. Thanks in advance.
  2. jcsd
  3. Nov 21, 2015 #2
    When dealing with |x| it often helps to use the piece wise expansion of it:
    =x, x>0
    = -x, x<0.
    If you were on the interval from [-L,L] you would have 2 integrals one from [-L,0] and one from [0,L]. However, since you are only interested in the function from [0,L] then $|x|^3$= $x^3$. To solve the integral you have integration by parts like normal.
  4. Nov 21, 2015 #3


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    Did you mean [itex]\int_0^L |x|^3 cos(\frac{n\pi x)}{L}dx[/itex] of [itex]\int_{L/2}^{Ll/2} |x|^3 cos(\frac{n\pi x)}{L}dx[/itex]? In the first, which is what you wrote, x is always non-negative so |x|= x. For the second, do the integrals from -L/2 to 0 and from 0 to L/2 separately.
  5. Nov 21, 2015 #4
    Yeah, that was my bad; the question wants me to find the fourier series for f(x)=|x|^3 on -L<x<L. So I think I can choose [itex]A_{n}=\frac{2}{L}\int_{0}^{L}x^{3}cos{\frac{n\pi x}{L}}dx[/itex] and then [itex]A_{0}=\frac{1}{L}\int_{0}^{L}x^{3}dx[/itex]?
  6. Nov 21, 2015 #5
    Thanks for your reply. I was unclear in the question statement; It asks me to find the fourier series for the function f(x)=|x|^3 on -L<x<L. I guess I can choose [itex]A_{n}=\frac{2}{L}\int_{0}^{L}x^{3}cos{\frac{n\pi x}{L}}dx[/itex] and then [itex]A_{0}=\frac{1}{L}\int_{0}^{L}x^{3}dx[/itex]?
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