Maxima Operator
!!The double factorial operator.
For an integer, float, or rational number n,
n!! evaluates to the product n (n-2) (n-4) (n-6) ... (n - 2 (k-1))
where k is equal to entier (n/2),
that is, the largest integer less than or equal to n/2.
Note that this definition does not coincide with other published definitions
for arguments which are not integers.
For an even (or odd) integer n, n!! evaluates to the product of
all the consecutive even (or odd) integers from 2 (or 1) through n inclusive.
For an argument n which is not an integer, float, or rational,
n!! yields a noun form genfact (n, n/2, 2).