<-- Previous || Up || Next -->

Gamma IT Function
Math Engineering Class

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

Calculate Tricomi's form of the incomplete Gamma function.

Examples: 
    GammaIT(1.0, 1.0) = 0.632120558828558
    GammaIT(1.0, 2.0) = 0.432332358381694
    GammaIT(2.0, 1.0) = 0.264241117657115
    GammaIT(2.0, 2.0) = 0.148498537572541
    GammaIT(2.2, 1.1) = 0.200054822983999
See also: 
    Gamma Function
    GammaI Function
    GammaIC Function
    GammaILn Function
    DGAMIT 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 Tricomi's incomplete Gamma function defined by 

    GammaIT = X^(-A)/GAMMA(A) * integral from 0 to X of EXP(-T) * T^(A-1.)
for A > 0 and by analytic continuation for A <= 0. GAMMA(X) is the complete gamma function of X.
GammaIT is evaluated for arbitrary real values of A and for non- negative values of X (even though GammaIT is defined for X < 0), except that for X = 0 and A <= 0, GammaIT is infinite, which is a fatal error.
A slight deterioration of 2 or 3 digits accuracy will occur when GammaIT is very large or very small in absolute value, because log- arithmic variables are used. Also, if the parameter A is very close to a negative integer (but not a negative integer), there is a loss of accuracy, which is reported if the result is less than half machine precision.

Copyright 1996-1999 Entisoft
Entisoft Tools is a trademark of Entisoft.