fac

Factorial function.

SYNOPSIS:

double y, fac();
int i;
y = fac(i);

DESCRIPTION:

Returns factorial of i = 1 * 2 * 3 * ... * i.
fac(0) = 1.0.

Due to machine arithmetic bounds the largest value of i accepted is 33 in DEC arithmetic or 170 in IEEE arithmetic. Greater values, or negative ones, produce an error message and return MAXNUM.

ACCURACY:

For i < 34 the values are simply tabulated, and have full machine accuracy. If i > 55, fac(i) = gamma(i+1);

Relative error:

arithmetic   domain      peak
    IEEE      0, 170    1.4e-15
    DEC       0, 33     1.4e-17