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:


 * $\displaystyle \psi \left({z}\right) = \frac{\Gamma'\left({z}\right)} {\Gamma\left({z}\right)}$

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