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Definition of Double factorial
1. Noun. (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. ¹
¹ Source: wiktionary.com
Lexicographical Neighbors of Double Factorial
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