39.2 Gamma Function (.NET, C#, CSharp, VB, Visual Basic, F#)
The static GammaLn()
method on class StatsFunctions
evaluates the log of the gamma function 
at a value x. The gamma function is an extension of
the factorial function to complex and real number arguments.
The "complete" gamma function 
can be generalized
to the incomplete gamma function 
, such that 
. The "lower" incomplete
gamma function is given by:
IncompleteGamma() returns the value of the lower regularized incomplete gamma function.