Evaluates the incomplete gamma function γ(x, y).

x—Parameter of the incomplete gamma function is to be evaluated. x must be positive.

y—Point at which the incomplete gamma function is to be evaluated. y must be nonnegative.

result—The value of the incomplete gamma function γ(x, y).

Double—If present and nonzero, double precision is used.

The incomplete gamma function, γ(x, y), is defined to be:

for y > 0. The incomplete gamma function is defined only for x > 0. Although γ(x, y) is well defined for y > −∞, this algorithm does not calculate γ(x, y) for negative y. For large x and sufficiently large y, γ(x, y) may overflow. γ(x, y) is bounded by Γ(x), and users may find this bound a useful guide in determining legal values for x.

Evaluates the incomplete gamma function at x = 1 and y = 3.

x  =  1.0

y  =  3.0

ans  =  GAMMAI(x, y)

PRINT, 'incomplete gamma(', x, ',', y, ') =', ans

; PV-WAVE prints: incomplete gamma( 1.00000, 3.00000) = 0.950213

STAT_NO_CONV_200_TS_TERMS—The function did not converge in 200 terms of Taylor series.

STAT_NO_CONV_200_CF_TERMS—The function did not converge in 200 terms of the continued fraction.