A special mathematical function related to the gamma function,
generalized for multivariate distributions.
The multivariate digamma function is the derivative of the log of the
multivariate gamma function; for \(p = 1\) it is the same as the
univariate digamma function.
$$\psi_{p}(a)=\sum _{i=1}^{p}\psi(a+(1-i)/2)
$$
where \(\psi\) is the univariate digamma function (the
derivative of the log-gamma function).