ASA032 is a FORTRAN77 library which evaluates the incomplete Gamma function, by G Bhattacharjee.
ASA032 is Applied Statistics Algorithm 32. Source code for many Applied Statistics Algorithms is available through STATLIB.
ASA032 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA147, a FORTRAN77 library which evaluates the incomplete Gamma function.
ASA239, a FORTRAN77 library which evaluates the incomplete Gamma function.
DCDFLIB, a FORTRAN90 library which evaluates and inverts a number of statistical distributions.
GSL, a C++ library which includes many routines for evaluating probability distributions.
PROB, a FORTRAN90 library which evaluates and inverts a number of probabilistic distributions.
TEST_VALUES, a FORTRAN77 library which contains sample values for a number of distributions.
TOMS435, a FORTRAN77 library which evaluates the modified incomplete Gamma function.
TOMS708, a FORTRAN77 library which includes routines to evaluate the incomplete Gamma function.
Original FORTRAN77 version by G Bhattacharjee; This FORTRAN77 version by John Burkardt.
You can go up one level to the FORTRAN77 source codes.