Holmstrom Functional equations

http://mathoverflow.net/questions/2040/why-are-functional-equations-important

http://mathoverflow.net/questions/14189/constants-in-gamma-factors-in-functional-equation-for-zeta-functions

arXiv:1001.1869 Analytic Continuation of some zeta functions from arXiv Front: math.NT by Gautami Bhowmik This is an expository paper on the meromorphic continuation of zeta functions with Euler products (for example zeta functions of groups and height zeta functions) or without (for example the Goldbach zeta function). As an application we show how a natural boundary of analytic continuation can give asymptotic results.

nLab page on Functional equations