# double factorial

## EnglishEdit

### EtymologyEdit

### NounEdit

**double factorial** (*plural* **double factorials**)

- (mathematics, combinatorics) For an odd integer, the result of multiplying all the odd integers from 1 to the given number; or for an even integer, the result of multiplying all the even integers from 2 to the given number; symbolised by a double exclamation mark (!!). For example, 9!! = 1 * 3 * 5 * 7 * 9 = 945.

#### External linksEdit

- MathWorld, Double Factorial