Cdigamma Digamma Polygamma Trigamma

Digamma() Function Digamma ( X As  Double, Optional Info As  Long  ) Digamma (or psi) function ψ(x) Purpose This routine computes the digamma (or psi) function ψ(x), where x is not 0, -1, -2, ... ψ(x) = d/dx ln(Γ(x)) = Γ'(x)/Γ(x) Returns Double Digamma (or psi) function ψ(x). Parameters [in] X Argument x. (X <> 0 and X <> a negative integer) [out] Info (Optional) = 0: Successful exit. = -1: The argument X had an illegal value. (X = 0 or a negative integer) = 1: Floating point range error. Reference boost/math/special_functions