Definition:Digamma Function

Definition
The digamma function, $\psi$, is defined, for $z \in \C \setminus \Z_{\le 0}$, by the logarithmic derivative of the gamma function:


 * $\map \psi z = \dfrac {\map {\Gamma'} z} {\map \Gamma z}$

where $\Gamma$ is the gamma function, and $\Gamma'$ denotes its derivative.