(HP71B) Modern GAMMA formula for Forth or ASM?

[John Keith]

More information on Stirling's approximation can be found here and here. More details here under "Implementation". Assuming that a reasonable number of terms are sufficient for 12-digit accuracy, it would be faster and easier to use in-line constants for the coefficients in your code.

The HP 49 and 50 have the Gamma function for real and complex arguments. I haven't tried to examine the internal code myself but I would guess that it is rather involved.