# Thread: Calculating a series

1. ## Calculating a series

I came across a problem that asked whether $\sum_{n=1}^\infty \sin(\pi\sqrt{n^2+1})$ converges or not. I was able to show that it converges (ask if you'd like to see why), but I was wanting to take this a step further:

What does $\sum_{n=1}^\infty \sin(\pi\sqrt{n^2+1})$ converge to? I'm under the impression that there might not be an answer expressible in terms of finite composition of elementary functions but I'm not sure.

Thanks everybody
-Chip

2. Well my guess would be to look for a fourier series, of a general term:
a_n cos(nx), where a_n equals your sine function, then I need to solve:
0=\int_{-\pi}^{\pi} sin(nx) f(x) dx
and a_n= \int_{-\pi}^{\pi} cos(nx)f(x) dx/N where N is the normalization constant, you could try using fourier transform as in:
g(n)=\int f(x) exp(-inx)dx (normalization factor is missing) from the above you can find what is g(n) and by inverse fourier transform you can find what is f. cause f(x)=\int g(n)exp(inx)dn...

This is theoretical, and maybe wrong approach, but that's what came to my mind, didn't do the calculations myself.