Gamma function

The computation of `gamma`

is based on algorithms outlined in [1].

[1] Cody, J., *An Overview of Software
Development for Special Functions*, Lecture Notes in Mathematics,
506, Numerical Analysis Dundee, G. A. Watson (ed.), Springer Verlag,
Berlin, 1976.

[2] Abramowitz, M. and I.A. Stegun, *Handbook
of Mathematical Functions*, National Bureau of Standards,
Applied Math. Series #55, Dover Publications, 1965, sec. 6.5.

`factorial`

| `gammainc`

| `gammaincinv`

| `gammaln`

| `psi`