— Function: double gsl_sf_psi_1_int (int n)
— Function: int gsl_sf_psi_1_int_e (int n, gsl_sf_result * result)

These routines compute the Trigamma function \psi'(n) for positive integer n.

— Function: double gsl_sf_psi_1 (double x)
— Function: int gsl_sf_psi_1_e (double x, gsl_sf_result * result)

These routines compute the Trigamma function \psi'(x) for general x.