Factorial2

The double factorial , the product of every second integer from n down to 1 or 2.

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Factorial2(n)the double factorial .

Domain: Combinatorics

Details

Skips every other factor: , stopping at 1 (n odd) or 2 (n even).
Recurrence: , with .
Splits an ordinary factorial into its even and odd parts: .
counts the perfect matchings of objects into pairs.
compute-engine's recurrence hits a pole at negative even integers (a division by the term), returning NaN instead of extending to negative odd arguments.

Examples

See also: Factorial