double factorial

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English[edit]

Etymology[edit]

double + factorial

Noun[edit]

double factorial (plural double factorials)

  1. (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.

Further reading[edit]