Functional equation for the zeta function
Introduction
The functional equation for Riemann zeta function is a result due to analytic continuation of Riemann zeta function:
![\[\zeta(s)=2^s\pi^{s-1}\sin\left(\pi s\over2\right)\Gamma(1-s)\zeta(1-s)\]](http://latex.artofproblemsolving.com/7/8/3/78380ea7c8198c2fc2ec42ee52af5523d1e286e9.png)
Proof
Two useful identities
There are multiple proofs for the functional equation for Riemann zeta function, and this page presents a light-weighted approach which merely relies on the Fourier series for the first periodic Bernoulli polynomial that
![\[B_1(x)\triangleq\{x\}-\frac12=-\sum_{n=1}^\infty{\sin(2\pi nx)\over\pi n}\]](http://latex.artofproblemsolving.com/a/8/3/a83075739a5adb1f2e8b34b99c53c6f4fe8c7ce5.png)
From 
to 
In this article, we will use the common convention that 
where 
. As a result, we say that the original Dirichlet series definition 
converges only for 
. However, if we were to apply Euler-Maclaurin summation on this definition, we obtain
![\[\zeta(s)=\frac12+{s\over s-1}-s\int_1^\infty{B_1(x)\over x^{s+1}}\mathrm dx\]](http://latex.artofproblemsolving.com/b/d/f/bdf72656f52fee99d364b45f27aeda2e657bad11.png)
in which we can extend the ROC of the latter integral to 
via integration by parts:
\begin{aligned}
\int_1^\infty{B_1(x)\over x^{s+1}}\mathrm dx
={B_2(x)\over2x^{s+1}}+{s+1\over2}\int_1^\infty{B_2(x)\over x^{s+2}}\mathrm dx
\end{aligned} (Error compiling LaTeX. Unknown error_msg)
