Friday, 20 November 2015

sequences and series - Using the residue theorem to evaluate sumlimitsn=inftyinftyfraceinalpha(nbeta)2+gamma2

I would like to know how to sum up to following series (from the Gradshteyn-Ryzhik tables):



n=einα(nβ)2+γ2=πγeiβ(α2π)sinh(γα)+eiβαsinh[γ(2πα)]cosh(2πγ)cos(2πβ)



with 0α2π. In the special case of α=0, we have n=1(nβ)2+γ2=πγsinh[2πγ]cosh(2πγ)cos(2πβ) and I now that I can use the function cot(πz)(zβ)2+γ2 to sum up this series via the residue theorem.
In more detail, the singularities are β±iγ and zn=n, nN.
If I sum the corresponding residues, I get what the above result (for α=0).




I am not sure, however:



1) How to choose the contour in order to have a correct argumentation?



2) What to do with the general case α0?

No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find lim without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...