Zeta[-13] == Zeta[-1] == -1/12  Why do these two different zetas produce the same value?  Answer  In order to understand the issue, we should provide the underlying definitions. Mathematica  helps in verifying appropriate relations and definitions. The main functional equation relating Riemann's zeta function $\zeta\;$, to Euler's $\Gamma\;$, established in Riemann's famous paper Über die Anzahl der Primzahlen unter einer gegebener Grösse  (1859, English translation here ), where he formulated the Riemann hypothesis, can be simply written and evaluated with the system: Through @ { HoldForm, FullSimplify}[              Zeta[z] == 2^z Pi^(z - 1) Sin[Pi z/2] Gamma[1 - z] Zeta[1 - z]]// Column // TraditionalForm $$\begin{align*}&\zeta(z)=2^z\pi^{-1+z}\Gamma(1-z)\sin\left(\frac{\pi z}{2}\right)\zeta(1-z)\\ &\text{True}\end{align*}$$ Similarly we can exploit the definition of Zeta  for Re[z] > 1  (see SumConvergence[1/n^z, n] ): Sum[ 1/n^z, {n, Infinity}] == Defer[ Sum...