**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

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