The incomplete gamma function.
Y = GAMMAINC(X,A) evaluates the incomplete gamma function at all
the elements of X. X must be real. A must be a real scalar.
gammainc(x,a) = (integral from 0 to x of t^(a-1) exp(-t) dt)/gamma(a).
Note that gammainc(x,a) approaches 1 as x approaches infinity.
See also GAMMA1LQPOWQ, GAMMALN .