GammaRelatedGammaI Method

Evaluate the incomplete gamma function defined by GammaI = integral from t = 0 to x of exp(-t) * t**(a-1.0) GammaI(x,a) is evaluated for positive values of a and non-negative values of x. A slight deterioration of 2 or 3 digits accuracy will occur when GammaI is very large or very small, because logarithmic variables are used. The function and both arguments are double precision.

public static double GammaI(
	double x,
	double a
)

This is a translation from the Fortran version of DGAMI, SLATEC, FNLIB,
CATEGORY C7E, REVISION 900315, originally written by Fullerton W.,(LANL)
to C++.

Routines called:
  LnGamma(x)      - logarithm of the Gamma function
  GammaIT(x,a)    - Tricomi's incomplete gamma function