ASA121 is a FORTRAN77 library which evaluates the trigamma function, by BE Schneider.
ASA121 is Applied Statistics Algorithm 121. Source code for many Applied Statistics Algorithms is available through STATLIB.
ASA121 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA103, a FORTRAN77 library which evaluates the digamma function.
TEST_VALUES, a FORTRAN90 library which contains sample values for a number of special functions.
Original FORTRAN77 version by BE Schneider; This FORTRAN77 version by John Burkardt.
You can go up one level to the FORTRAN77 source codes.