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Gamma I Ln Function
Math Engineering Class

Public Function GammaILn( _
      ByVal vA As Variant _
    , ByVal vX As Variant _
    ) As Variant

Evaluate the log of the incomplete Gamma function.

Examples: 
    GammaILn(1.0, 1.0) = -0.458675145387082
    GammaILn(1.0, 2.0) = -0.145413457868859
    GammaILn(2.0, 1.0) = -1.33089326820405
    GammaILn(2.0, 2.0) = -0.520885807664344
    GammaILn(2.2, 1.1) = -1.30253397271658
    GammaILn(2.2, 1.1) = Log(GammaI(2.2, 1.1))
See also: 
    GammaLn Function
    GammaI Function
    Gamma Function
    DLNGMI Function
vA: Function returns Null if vA is Null or cannot be fixed up to a Double precision floating point number.
vX: Function returns Null if vX is Null or cannot be fixed up to a Double precision floating point number.

Evaluate the log of the incomplete gamma function defined by 

    GammaILn = log of integral from T = 0 to X of EXP(-T) * T^(A-1.0)
GammaILn 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 GammaILn is very large or very small, because logarithmic variables are used. The function and both arguments are double precision.

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