# Homework Help: Series expansion of Coth(pi)

1. Aug 29, 2012

### Ratpigeon

1. The problem statement, all variables and given/known data

show coth($\pi$)=1/$\pi$ (1+2 $\sum$$\infty$n=1 (1/(1+n^2)

2. Relevant equations
The fourier expansion of e^x is
Sinh($\pi$)/$\pi$(1+$\sum$ 2(-1)^m/(1+n^2) (cos(mx)-n sin(nx)

3. The attempt at a solution
I subbed in
Coth(Pi)=1+e^-pi/sinh(pi) =1+1/$\pi$ (1+2 $\sum$$\infty$n=1 (1/(1+n^2)

But this is wrong because of the one out the front - I know that needs to stay there, which means I somehow need to stick a -sinh(pi) into the front of my fourier series of e^pi I would say that it would be my term for n= zero, but I already took care of that one, which was sinh(pi)/pi...

2. Aug 29, 2012

### vela

Staff Emeritus
You're missing closing parentheses and have two index variables, m and n, so I'm not sure what you meant to write exactly. Did you mean
$$\frac{\sinh \pi}{\pi}\left(1+\sum_{n=1}^\infty \frac{2(-1)^n}{1+n^2}(\cos nx - n\sin nx)\right)?$$

How'd you come up with $\coth \pi = 1+\frac{e^{-\pi}}{\sinh \pi}$?

Last edited: Aug 29, 2012
3. Aug 29, 2012

### Ratpigeon

Coth($\pi$) =(e$\pi$+e-$\pi$)/(e$\pi$-e-$\pi$)

which equals
1+2e-$\pi$/(e$\pi$-e-$\pi$)
which equals
1+2e^pi/2sinh(pi)

4. Aug 29, 2012

### gabbagabbahey

As vela pointed out, this makes no sense. What is the actual Fourier series expansion of $e^x$?

Last edited: Aug 29, 2012
5. Aug 29, 2012

### Ratpigeon

It's the one that vela used - I usually use n's but the lecturer uses m's so it's a bit messed up, and they are both standing for the same thing. And the e^pi is a typo - I have it right in my working...
I've found another page that uses this as the fourier expansion of e^x, so I'm fairly sure that it's right (after making all the indeces the same)

6. Aug 29, 2012

### vela

Staff Emeritus
Hint: What does your Fourier series converge to when you set $x=\pi$? Keep in mind there's a discontinuity there.

7. Aug 29, 2012

### Ratpigeon

It'll be... halfway between the limit of the fourier series at Pi and -pi?
Which means....
1/2(e^pi+e^-pi)=Sn(+/-pi)?

8. Aug 29, 2012

### vela

Staff Emeritus
I don't know what you mean by "Sn(+/-pi)", but yeah, it'll be the average.

9. Aug 29, 2012

### Ratpigeon

And that lets me calculate e^pi+e^-pi from my fourier series at Pi; but how do I get e^pi-e^-pi for the denominator then?

10. Aug 29, 2012

### vela

Staff Emeritus
Here's another hint: What's $\cosh \pi$ equal to?

11. Aug 29, 2012

### Ratpigeon

wait, no, I got it. The denominator is 2 sinh(pi) and the numerator is twice the fourier series at Pi - which is sinH(pi) multiplied by the series I want.
Thanks!
:)