Returns a list with the values of the lower, the upper, and regularized
lower incomplete gamma function.
Details
Computes the lower and upper incomplete gamma function, including the
regularized gamma function. The lower and upper incomplete gamma functions
are defined as
$$\gamma(x, a) = \int_0^x e^{-t} \, t^{a-1} \, dt$$
and
$$\Gamma(x, a) = \int_x^{\infty} e^{-t} \, t^{a-1} \, dt$$
while the regularized incomplete gamma function is
$\gamma(x, a)/\Gamma(a)$.
Accuracy is 7 significant digits along the real axis.
References
Zhang, Sh., and J. Jin (1996). Computation of Special Functions.
Wiley-Interscience, New York.