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Gamma I Function
Math Engineering Class

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

Evaluate the incomplete Gamma function.

Examples: 
    GammaI(1.0, 1.0) = 0.632120558828558
    GammaI(1.0, 2.0) = 0.864664716763387
    GammaI(2.0, 1.0) = 0.264241117657115
    GammaI(2.0, 2.0) = 0.593994150290162
    GammaI(2.2, 1.1) = 0.271842079133223
See also: 
    Gamma Function
    GammaIC Function
    GammaIT Function
    GammaILn Function
    GammaR Function
    GammaCDF Function
    GammaInverse Function
    DGAMI 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 incomplete gamma function defined by 

    GammaI = integral from T = 0 to X of EXP(-T) * T^(A-1.0)
GammaI 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.

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