The FACTDOUBLE function calculates the double factorial of a number n. It takes into account whether n is even or odd, and then carries out a factorial calculation using either the even or the odd numbers up to n.
For example:
FACTDOUBLE(5) returns 15, that is 5 * 3 * 1.
FACTDOUBLE(6) returns 48, that is 6 * 4 * 2.
FACTDOUBLE also works with fractional numbers (see FACT). For example:
FACTDOUBLE(5.5) returns 28.875, that is 5.5 * 3.5 * 1.5.
FACTDOUBLE(6.5) returns 73.125, that is 6.5 x 4.5 x 2.5.
See also: