digamma() [4/7]

double stan::math::digamma ( double  x )

inline

Return the derivative of the log gamma function at the specified value.

$$\mbox{digamma}(x) = \begin{cases} \textrm{error} & \mbox{if } x\in \{\dots, -3, -2, -1, 0\}\\ \Psi(x) & \mbox{if } x\not\in \{\dots, -3, -2, -1, 0\}\\[6pt] \textrm{NaN} & \mbox{if } x = \textrm{NaN} \end{cases}$$

$$\frac{\partial\, \mbox{digamma}(x)}{\partial x} = \begin{cases} \textrm{error} & \mbox{if } x\in \{\dots, -3, -2, -1, 0\}\\ \frac{\partial\, \Psi(x)}{\partial x} & \mbox{if } x\not\in \{\dots, -3, -2, -1, 0\}\\[6pt] \textrm{NaN} & \mbox{if } x = \textrm{NaN} \end{cases}$$

$$\Psi(x)=\frac{\Gamma'(x)}{\Gamma(x)}$$

$$\frac{\partial \, \Psi(x)}{\partial x} = \frac{\Gamma''(x)\Gamma(x)-(\Gamma'(x))^2}{\Gamma^2(x)}$$

The design follows the standard C++ library in returning NaN rather than throwing exceptions.

Parameters

[in]

x

argument

Returns

derivative of log gamma function at argument

Definition at line 47 of file digamma.hpp.