Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sum (sin(Pi*n)/(-1+n^2) , n=1infinity). n=1,2,3

  1. Oct 3, 2011 #1
    Hi everybody!!

    Solving a PDE for the damped wave equation I get with a solutions that part of it have the expression:

    Sum (sin(Pi*n)/(-1+n^2) , n=1..infinity). n=1,2,3...

    When calculating it with Maple I get this is equal to -1/2*Pi.

    Can someone of you explain me why?

    Thanks a lot!!
     
  2. jcsd
  3. Oct 3, 2011 #2

    LCKurtz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Did you get that expression from a Fourier Series expansion? Sometimes these sums arise by simply evaluating the FS at some point.
     
  4. Oct 3, 2011 #3
    Yes, I'm trying to solve the PDE for the damped wave equations defined like:

    u_tt+u_t-u_xx=0,

    With Initial Conditions: u(x,0)=0 , u_t (x,0)=sin⁡(x)
    And Boundary Conditions u(0,t)=u(Pi,t)=0

    Finally I end with the solution u(x,t)=∑(n=1 to ∞) [exp^(-t/2)*(-4sin⁡(Pi*n))*sin⁡(sqrt(4n^2-1)*t/2)*sin(nx) / (Pi*sqrt(4n^2-1)*(-1+n^2))]

    From here, there is the expression sin(Pi*n)/(-1+n^2), that for n=1 is 0. But solving it with Maple it gives u(x,t)=2/(3*sqrt(3))*exp^(-t/2)*sin(sqrt(3)*t/2)*sin(x).

    I've also found that limit [when n-->1] of sin(Pi*n)/(-1+n^2) = -Pi/2; that plugged on the series give what Maple says.

    But I'm still not sure if it is correct or not

    Thanks!!!
     
  5. Oct 3, 2011 #4

    Mute

    User Avatar
    Homework Helper

    [itex]\sin(n\pi)[/itex] is zero for any integer n. So, all of your terms except for the first one is zero. The first is treated as non-zero because the denominator vanishes at n = 1. So, you use L'Hopital's rule to evaluate it instead.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Sum (sin(Pi*n)/(-1+n^2) , n=1infinity). n=1,2,3
  1. M - (x -n)^2 = ? (Replies: 16)

  2. Egyptian 2/n-table (Replies: 10)

  3. Mth digit of 2^n (Replies: 10)

Loading...