I am referring to the equality in equation $3.29$ (page 12) and $4.20$ (page 17) in this paper.
I am unable to recognize where this comes from or what is the general expression for values other than $3$.
I checked at some online reviews like this - http://www.math.utah.edu/~milicic/zeta.pdf but nothing seems to match.
It would be great if someone can help.
(Images added by J.O'Rourke)