ASA109 is a C 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.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
ASA109 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
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Original FORTRAN77 version by GW Cran, KJ Martin, GE Thomas; C version by John Burkardt.
You can go up one level to the C source codes.