!)Factorial (SymPy, mpmath, WMA)
Factorial[n]n!20!
Factorial handles numeric (real and complex) values using the gamma function:
10.5!
(-3.0+1.5*I)!
However, the value at poles is ComplexInfinity:
(-1.)!
Factorial has the same operator (!) as Not, but with higher precedence:
!a! //FullForm