Description

These functions compute the gamma function and its natural logarithm. Both work with double precision floating point numbers and return a result of that type.

:  tgamma	true (Euler) gamma function: tgamma(n+1) = n! for non-negative integer n.
:  lgamma	natural logarithm of the absolute value of tgamma(x); usable where tgamma would overflow.
    

For large arguments tgamma overflows quickly; lgamma remains finite over the entire representable range and is the correct choice when only the order of magnitude is needed.