This series is given in Griffiths Introduction to Electrodynamics chapter 3 in an example explaining seperation of variables.

$$V(x,y) = \frac{4V_0}{\pi} \sum_{n=1,3,5\ldots} \frac{1}{n}e^{-n\pi x/a}\sin(n\pi y/a)$$ $$V(x,y) = \frac{2V_0}{\pi} \tan^{-1}\left(\frac{\sin(\pi y/a)}{\sinh(\pi x/a)} \right)$$

I don't have any idea how to get the analytic representation of the series. I tried replacing $n$ by $ -n$ to get hyperbolic sine but it also introduces summation over negative odd integers and I don't think they can be added over. Also, it will form hyperbolic sine in the numerator rather than the denominator. How to proceed?

  • 1
    $\begingroup$ Replacing $n$ by $-n$ doesn’t make the sine turn into a hyperbolic sine. $\endgroup$
    – G. Smith
    Jul 22, 2019 at 16:07
  • 1
    $\begingroup$ A sum over negative integers is perfectly valid, but I don't think it helps here. $\endgroup$
    – G. Smith
    Jul 22, 2019 at 16:09
  • 1
    $\begingroup$ In the old days one spend a lot more time poring over math tables and just knew which section to go look in... $\endgroup$
    – Jon Custer
    Jul 23, 2019 at 20:31
  • 1
    $\begingroup$ Put $n=2k-1$, use $\sin\,x =\frac {e^{ix}-e^{-ix}} {2i}$ and you will end up with a series of the type $\sum \frac {r^{k}} {2k-1}$ with $|r| <1$. Can you handle this series? $\endgroup$ Jul 26, 2019 at 23:13
  • $\begingroup$ Yeah, it's solved. Thanks. $\endgroup$ Jul 27, 2019 at 6:29


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.