gammaIncompleteComplInv

Inverse of complemented incomplete gamma integral

Given a and p, the function finds x such that

gammaIncompleteCompl( a, x ) = p.

Starting with the approximate value x = a t³, where t = 1 - d - normalDistributionInv(p) sqrt(d), and d = 1/9a, the routine performs up to 10 Newton iterations to find the root of incompleteGammaCompl(a,x) - p = 0.