FNIgammareturns the value of the incomplete gamma function ofaandx,γ(a,x). Although the incomplete gamma function is defined for all values ofx, this subroutine only works with values ofx> 0 orx= 0 anda> 0.
γ(a,x) is defined by the expression
In the above expression,xmust be positive or zero andamust be positive.
Whenx≥0 anda> 0, the other forms of the incomplete gamma function present in this subroutine library, the complementary form,Γ(a,x) and Tricomi's form,γ*(a,x), are related to the incomplete gamma function by the following expressions:
Errors
FNIgammacauses an HTBasic error if its arguments are not both of type REAL, ifx< 0, or ifx= 0 anda≤0.
