FACT(x)

Returns the factorial of a number. The factorial of a number is equal to 1*2*3*...* number. If the number is not an integer, it is truncated. A negative argument will generate an error. Note that if used alone (as in Y=fact(X)), fact will cause a trapped overflow with an argument greater than or equal to 171.

Example:
FACT(4)=24
FACT(20)=2.342902E+18