cdfgam - cumulative distribution function gamma distribution
Calculates any one parameter of the gamma distribution given values for the others.
Cumulative distribution function (P) is calculated directly by the code associated with:
DiDinato, A. R. and Morris, A. H. Computation of the incomplete gamma function ratios and their inverse. ACM Trans. Math. Softw. 12 (1986), 377-393.
Computation of other parameters involve a seach for a value that produces the desired value of P. The search relies on the monotinicity of P with the other parameter.
The gamma density is proportional to T**(SHAPE - 1) * EXP(- SCALE * T)
From DCDFLIB: Library of Fortran Routines for Cumulative Distribution Functions, Inverses, and Other Parameters (February, 1994) Barry W. Brown, James Lovato and Kathy Russell. The University of Texas.