1. Algorithm 610: A portable FORTRAN subroutine for derivatives of the psi function;D E Amos;ACM Transactions on Mathematical Software,1983
2. MAPLE SYRUP
3. Algorithm AS 103: Psi (digamma) function;J M Bernardo;Applied Statistics,1976
4. Estimation and inference in non-linear structural models;E K Berndt;Annals of Economic and Social Measurement,1974