# §25.5(i) In Terms of Elementary Functions

Throughout this subsection $s\neq 1$.

 25.5.1 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{1}{\mathop{\Gamma\/}\nolimits\!\left(s\right)}\int_{0}^{% \infty}\frac{x^{s-1}}{e^{x}-1}dx,$ $\realpart{s}>1$. 25.5.2 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{1}{\mathop{\Gamma\/}\nolimits\!\left(s+1\right)}\int_{0}^{% \infty}\frac{e^{x}x^{s}}{(e^{x}-1)^{2}}dx,$ $\realpart{s}>1$.
 25.5.3 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{1}{(1-2^{1-s})\mathop{\Gamma\/}\nolimits\!\left(s\right)}% \int_{0}^{\infty}\frac{x^{s-1}}{e^{x}+1}dx,$ $\realpart{s}>0$. 25.5.4 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{1}{(1-2^{1-s})\mathop{\Gamma\/}\nolimits\!\left(s+1\right)% }\int_{0}^{\infty}\frac{e^{x}x^{s}}{(e^{x}+1)^{2}}dx,$ $\realpart{s}>0$.
 25.5.5 $\mathop{\zeta\/}\nolimits\!\left(s\right)=-s\int_{0}^{\infty}\frac{x-\left% \lfloor x\right\rfloor-\frac{1}{2}}{x^{s+1}}dx,$ $-1<\realpart{s}<0$.
 25.5.6 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{1}{2}+\frac{1}{s-1}+\frac{1}{% \mathop{\Gamma\/}\nolimits\!\left(s\right)}\int_{0}^{\infty}\left(\frac{1}{e^{% x}-1}-\frac{1}{x}+\frac{1}{2}\right)\frac{x^{s-1}}{e^{x}}dx,$ $\realpart{s}>-1$.
 25.5.7 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{1}{2}+\frac{1}{s-1}+\sum_{m=1}% ^{n}\frac{\mathop{B_{2m}\/}\nolimits}{(2m)!}\frac{\mathop{\Gamma\/}\nolimits\!% \left(s+2m-1\right)}{\mathop{\Gamma\/}\nolimits\!\left(s\right)}+\frac{1}{% \mathop{\Gamma\/}\nolimits\!\left(s\right)}\int_{0}^{\infty}\left(\frac{1}{e^{% x}-1}-\frac{1}{x}+\frac{1}{2}-\sum_{m=1}^{n}\frac{\mathop{B_{2m}\/}\nolimits}{% (2m)!}x^{2m-1}\right)\frac{x^{s-1}}{e^{x}}dx,$ $\realpart{s}>-(2n+1)$, $n=1,2,3,\dots$.
 25.5.8 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{1}{2(1-2^{-s})\mathop{\Gamma\/}\nolimits\!\left(s\right)}% \int_{0}^{\infty}\frac{x^{s-1}}{\mathop{\sinh\/}\nolimits x}dx,$ $\realpart{s}>1$. 25.5.9 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{2^{s-1}}{\mathop{\Gamma\/}\nolimits\!\left(s+1\right)}\int% _{0}^{\infty}\frac{x^{s}}{(\mathop{\sinh\/}\nolimits x)^{2}}dx,$ $\realpart{s}>1$.
 25.5.10 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{2^{s-1}}{1-2^{1-s}}\int_{0}^{% \infty}\frac{\mathop{\cos\/}\nolimits\!\left(s\mathop{\mathrm{arctan}\/}% \nolimits x\right)}{(1+x^{2})^{s/2}\mathop{\cosh\/}\nolimits\!\left(\frac{1}{2% }\pi x\right)}dx.$
 25.5.11 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{1}{2}+\frac{1}{s-1}+2\int_{0}^% {\infty}\frac{\mathop{\sin\/}\nolimits\!\left(s\mathop{\mathrm{arctan}\/}% \nolimits x\right)}{(1+x^{2})^{s/2}(e^{2\pi x}-1)}dx.$
 25.5.12 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{2^{s-1}}{s-1}-2^{s}\int_{0}^{% \infty}\frac{\mathop{\sin\/}\nolimits\!\left(s\mathop{\mathrm{arctan}\/}% \nolimits x\right)}{(1+x^{2})^{s/2}(e^{\pi x}+1)}dx.$

# §25.5(ii) In Terms of Other Functions

 25.5.13 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{\pi^{s/2}}{s(s-1)\mathop{% \Gamma\/}\nolimits\!\left(\frac{1}{2}s\right)}+\frac{\pi^{s/2}}{\mathop{\Gamma% \/}\nolimits\!\left(\frac{1}{2}s\right)}\*\int_{1}^{\infty}\left(x^{s/2}+x^{(1% -s)/2}\right)\frac{\omega(x)}{x}dx,$ $s\neq 1$,

where

 25.5.14 $\omega(x)=\sum_{n=1}^{\infty}e^{-n^{2}\pi x}=\frac{1}{2}\left(\mathop{\theta_{% 3}\/}\nolimits\!\left(0\middle|ix\right)-1\right).$

For $\mathop{\theta_{3}\/}\nolimits$ see §20.2(i). For similar representations involving other theta functions see Erdélyi et al. (1954a, p. 339).

In (25.5.15)–(25.5.19), $0<\realpart{s}<1$, $\mathop{\psi\/}\nolimits\!\left(x\right)$ is the digamma function, and $\EulerConstant$ is Euler’s constant (§5.2). (25.5.16) is also valid for $0<\realpart{s}<2$, $s\neq 1$.

 25.5.15 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{1}{s-1}+\frac{\mathop{\sin\/}% \nolimits\!\left(\pi s\right)}{\pi}\*\int_{0}^{\infty}(\mathop{\ln\/}\nolimits% \!\left(1+x\right)-\mathop{\psi\/}\nolimits\!\left(1+x\right))x^{-s}dx,$
 25.5.16 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(s\right)$ $\displaystyle=\frac{1}{s-1}+\frac{\mathop{\sin\/}\nolimits\!\left(\pi s\right)% }{\pi(s-1)}\*\int_{0}^{\infty}\left(\frac{1}{1+x}-{\mathop{\psi\/}\nolimits^{% \prime}}\!\left(1+x\right)\right)x^{1-s}dx,$ 25.5.17 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(1+s\right)$ $\displaystyle=\frac{\mathop{\sin\/}\nolimits\!\left(\pi s\right)}{\pi}\int_{0}% ^{\infty}\left(\EulerConstant+\mathop{\psi\/}\nolimits\!\left(1+x\right)\right% )x^{-s-1}dx,$
 25.5.18 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(1+s\right)$ $\displaystyle=\frac{\mathop{\sin\/}\nolimits\!\left(\pi s\right)}{\pi s}\int_{% 0}^{\infty}{\mathop{\psi\/}\nolimits^{\prime}}\!\left(1+x\right)x^{-s}dx,$ 25.5.19 $\displaystyle\mathop{\zeta\/}\nolimits\!\left(m+s\right)$ $\displaystyle=(-1)^{m-1}\frac{\mathop{\Gamma\/}\nolimits\!\left(s\right)% \mathop{\sin\/}\nolimits\!\left(\pi s\right)}{\pi\mathop{\Gamma\/}\nolimits\!% \left(m+s\right)}\*\int_{0}^{\infty}{\mathop{\psi\/}\nolimits^{(m)}}\!\left(1+% x\right)x^{-s}dx,$ $m=1,2,3,\dots$.

# §25.5(iii) Contour Integrals

 25.5.20 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{\mathop{\Gamma\/}\nolimits\!% \left(1-s\right)}{2\pi i}\int_{-\infty}^{(0+)}\frac{z^{s-1}}{e^{-z}-1}dz,$ $s\neq 1,2,\dots$,

where the integration contour is a loop around the negative real axis; it starts at $-\infty$, encircles the origin once in the positive direction without enclosing any of the points $z=\pm 2\pi i$, $\pm 4\pi i$, …, and returns to $-\infty$. Equivalently,

 25.5.21 $\mathop{\zeta\/}\nolimits\!\left(s\right)=\frac{\mathop{\Gamma\/}\nolimits\!% \left(1-s\right)}{2\pi i(1-2^{1-s})}\*\int_{-\infty}^{(0+)}\frac{z^{s-1}}{e^{-% z}+1}dz,$ $s\neq 1,2,\dots$.

The contour here is any loop that encircles the origin in the positive direction not enclosing any of the points $\pm\pi i$, $\pm 3\pi i$, ….