i.org/10.1145/22721.23109">Didonato and Morris (1986),
Computation of the Incomplete Gamma Function Ratios and their Inverse, TOMS 12(4), 377-393,
Didonato and Morris (1992), Algorithm 708: Significant Digit Computation of the Incomplete Beta Function Ratios, TOMS 18(3), 360-373, and implemented in the
NSWC Library of Mathematical Functions, available
here. This library is "approved for public release", and the
Copyright guidance indicates that unless otherwise stated in the code, all FORTRAN functions in this library are license free. Since no such notice appears in the code these functions can safely be ported to Commons-Math.