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Homework Help: Orthogonal Properties for Sine Don't Hold if Pi is involded?

  1. Apr 3, 2010 #1
    Orthogonal Properties for Sine Don't Hold if Pi is involded??

    Normally I know [tex]
    \int_{-L}^L \sin \frac{n x}{L} \sin \frac{\m x}{L} ~ dx = 0\mbox{ if }n\not =m , \ =L \mbox{ if }n=m
    [/tex] but apparently this doesn't work for [tex]
    \int_{-L}^L \sin \frac{\pi n x}{L} \sin \frac{\pi m x}{L} ~ dx
    [/tex]

    I am trying to find fourier coefficients, and my integral is [tex]\int_{0}^{L/2} \sin \frac{2\pi x}{L} \sin \frac{\pi n x}{L} ~ dx
    [/tex]

    For even n that aren't 2, the integral is 0, if n=2, then the integral is L/2, but if n is odd, then the integral doesn't equal 0 (it's actually a fairly complex answer) even thought it looks like it should by orthogonal properties ...why doesn't this work?

    The integrals I am doing take very long to do, is there a property that would allow me to shorten them?
     
    Last edited: Apr 3, 2010
  2. jcsd
  3. Apr 3, 2010 #2

    LCKurtz

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    Re: Orthogonal Properties for Sine Don't Hold if Pi is involded??

    It does work. If [itex]n \ne m[/itex] are positive integers then

    [tex]\int_0^L\ \sin(\frac{n\pi x}{L})\sin(\frac{m\pi x}{L})\, dx = 0[/tex]

    Sturm-Liouville theory would guarantee this without working it out.
     
  4. Apr 3, 2010 #3
    Re: Orthogonal Properties for Sine Don't Hold if Pi is involded??

    Sorry the limits are actually 0 to L/2, in this case it doesn't work, is there another property that would help evaluate the integral quicker or do I have to go through the whole thing every time?
     
  5. Apr 3, 2010 #4

    LCKurtz

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    Re: Orthogonal Properties for Sine Don't Hold if Pi is involded??

    The set of functions

    [tex]\left \{ \sin\frac{2n\pi x}{L}\right\}[/tex]

    are orthogonal on [0, L/2].
     
  6. Apr 3, 2010 #5
    Re: Orthogonal Properties for Sine Don't Hold if Pi is involded??

    Is there anything like that for [tex]
    \left \{ \sin\frac{(2n)\pi x}{L}\right\ \left \sin\frac{(2n+1)\pi x}{L}\right\}
    [/tex]?
     
  7. Apr 3, 2010 #6

    LCKurtz

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    Re: Orthogonal Properties for Sine Don't Hold if Pi is involded??

    I doubt you will get zero, but why don't you just integrate it and see?
     
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