Stirling

Stirling's approximation to factorials.

Stirling´s approximation returns the logarithm of the factorial value or the factorial value for n as large as 170 (a greater value returns INF for it exceeds the largest floating point number, e+308). For a better expansion it is used the Kemp (1989) and Tweddle (1984) suggestions.
It needs to input n - can be a fractional or a scalar and o - option of the result (logarithm of the factorial value = 1; factorial value = 2)[default = 2].