InverseGammaRegularized[a, s] gives the inverse of the regularized incomplete gamma function.
Mathematical function (see Section A.3.10).
With the regularized incomplete gamma function defined by , InverseGammaRegularized[a, s] is the solution for in .
InverseGammaRegularized[a, , s] gives the inverse of GammaRegularized[a, , z].
Note that the arguments of InverseGammaRegularized are arranged differently than in InverseBetaRegularized.
See The Mathematica Book: Section 3.2.10.
See also: InverseBetaRegularized, InverseErf.