ASA109 is a FORTRAN90 library which "inverts" the incomplete Beta function, by Cran, Martin and Thomas.
Given a value of the incomplete Beta function, it can determine the argument that produced that value.
ASA109 is Applied Statistics Algorithm 109. Source code for many Applied Statistics Algorithms is available through STATLIB.
ASA109 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA063, a FORTRAN90 library which evaluates the incomplete Beta function.
ASA226, a FORTRAN90 library which evaluates the CDF of the noncentral Beta distribution.
ASA310, a FORTRAN90 library which computes the CDF of the noncentral Beta distribution.
BETA_NC, a FORTRAN90 library which evaluates the CDF of the noncentral Beta distribution.
TEST_VALUES, a FORTRAN90 library which stores a few values of various mathematical functions.
TOMS179, a FORTRAN90 library which evaluates the incomplete Beta function.
TOMS708, a FORTRAN90 library which evaluates the incomplete Beta function.
Original FORTRAN77 version by GW Cran, KJ Martin, GE Thomas; FORTRAN90 version by John Burkardt.
You can go up one level to the FORTRAN90 source codes.