# §5.7 Series Expansions

## §5.7(i) Maclaurin and Taylor Series

Throughout this subsection $\mathop{\zeta\/}\nolimits\!\left(k\right)$ is as in Chapter 25.

 5.7.1 $\frac{1}{\mathop{\Gamma\/}\nolimits\!\left(z\right)}=\sum_{k=1}^{\infty}c_{k}z% ^{k},$ Symbols: $\mathop{\Gamma\/}\nolimits\!\left(\NVar{z}\right)$: gamma function, $k$: nonnegative integer, $z$: complex variable and $c_{k}$: coefficient A&S Ref: 6.1.34 Referenced by: §5.7(i) Permalink: http://dlmf.nist.gov/5.7.E1 Encodings: TeX, pMML, png

where $c_{1}=1$, $c_{2}=\EulerConstant$, and

 5.7.2 $(k-1)c_{k}=\EulerConstant c_{k-1}-\mathop{\zeta\/}\nolimits\!\left(2\right)c_{% k-2}+\mathop{\zeta\/}\nolimits\!\left(3\right)c_{k-3}-\dots+(-1)^{k}\mathop{% \zeta\/}\nolimits\!\left(k-1\right)c_{1},$ $k\geq 3$.

For 15D numerical values of $c_{k}$ see Abramowitz and Stegun (1964, p. 256), and for 31D values see Wrench (1968).

 5.7.3 $\displaystyle\mathop{\ln\/}\nolimits\mathop{\Gamma\/}\nolimits\!\left(1+z\right)$ $\displaystyle=-\mathop{\ln\/}\nolimits\!\left(1+z\right)+z(1-\EulerConstant)+% \sum_{k=2}^{\infty}(-1)^{k}(\mathop{\zeta\/}\nolimits\!\left(k\right)-1)\frac{% z^{k}}{k},$ $|z|<2$. 5.7.4 $\displaystyle\mathop{\psi\/}\nolimits\!\left(1+z\right)$ $\displaystyle=-\EulerConstant+\sum_{k=2}^{\infty}(-1)^{k}\mathop{\zeta\/}% \nolimits\!\left(k\right)z^{k-1},$ $|z|<1$, 5.7.5 $\displaystyle\mathop{\psi\/}\nolimits\!\left(1+z\right)$ $\displaystyle=\frac{1}{2z}-\frac{\pi}{2}\mathop{\cot\/}\nolimits\!\left(\pi z% \right)+\frac{1}{z^{2}-1}+1-\EulerConstant-\sum_{k=1}^{\infty}(\mathop{\zeta\/% }\nolimits\!\left(2k+1\right)-1)z^{2k},$ $|z|<2$, $z\neq 0,\pm 1$.

For 20D numerical values of the coefficients of the Maclaurin series for $\mathop{\Gamma\/}\nolimits\!\left(z+3\right)$ see Luke (1969b, p. 299).

## §5.7(ii) Other Series

When $z\neq 0,-1,-2,\dots$,

 5.7.6 $\mathop{\psi\/}\nolimits\!\left(z\right)=-\EulerConstant-\frac{1}{z}+\sum_{k=1% }^{\infty}\frac{z}{k(k+z)}=-\EulerConstant+\sum_{k=0}^{\infty}\left(\frac{1}{k% +1}-\frac{1}{k+z}\right),$

and

 5.7.7 $\mathop{\psi\/}\nolimits\!\left(\frac{z+1}{2}\right)-\mathop{\psi\/}\nolimits% \!\left(\frac{z}{2}\right)=2\sum_{k=0}^{\infty}\frac{(-1)^{k}}{k+z}.$

Also,

 5.7.8 $\imagpart{\mathop{\psi\/}\nolimits\!\left(1+iy\right)}=\sum_{k=1}^{\infty}% \frac{y}{k^{2}+y^{2}}.$