Factorial​(System.​Numerics.​Complex)

x!, the gamma function one along. At a whole number up to 170 it is the exact
factorial rounded once, and past 170, where a double cannot hold it, it is infinite. At a
negative whole number, one of the gamma function's poles, it is NaN, as the interpreter's
is. Elsewhere it is Lanczos's approximation with g = 7 and nine terms, the one the
compiler has always used. Its t^(z + 1/2) e^(-t) is taken in logarithms, so that
it overflows only where the value does: the power alone passed the largest double just past
141, where the factorial is about 2e243, and the compiled factorial was NaN from
there to 170.
https://github.com/asc-community/AngouriMath/issues/1607