This function evaluates the incomplete gamma function.
Function Return Value
GAMI — Function value. (Output)
Required Arguments
A — The integrand exponent parameter. (Input) It must be positive.
X — The upper limit of the integral definition of GAMI. (Input) It must be nonnegative.
FORTRAN 90 Interface
Generic: GAMI (A, X)
Specific: The specific interface names are S_GAMI and D_GAMI.
FORTRAN 77 Interface
Single: GAMI (A, X)
Double: The double precision function name is DGAMI.
Description
The incomplete gamma function is defined to be
The function (a, x) is defined only for a greater than zero. Although (a, x) is well defined for x >‑∞, this algorithm does not calculate (a, x) for negative x. For large a and sufficiently large x, (a, x) may overflow. (a, x) is bounded by Γ(a), and users may find this bound a useful guide in determining legal values of a.
Because logarithmic variables are used, a slight deterioration of two or three digits of accuracy will occur when GAMI is very large or very small.
Figure 4.3 — Contour Plot of γ(a, x)
Example
In this example, (2.5, 0.9) is computed and printed.
