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 "upper" incomplete gamma function is given by:
IncompleteGamma() returns the value of the incomplete gamma function.
at a value 
can be generalized to the incomplete gamma function 
, such that 
. The "upper" incomplete gamma function is given by:
