double factorial

名词 n.
中文释义

英文释义

名词 n.
  1. 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.
    — The symbolical representation of the results of this paper is much facilitated by the introduction of a separate symbol for the product of alternate factors, n#92;cdotn-2#92;cdotn-4#92;cdots 1, if n be odd, or n#92;cdotn-2#92;cdots 2 if n be odd^([sic – meaning even]). I propose to write n#33;#33; for such products, and if a name be required for the product to call it the "alternate factorial" or the "double factorial." Full advantage of the new symbol is only gained by extending its meaning to the negative values of n. Its complete definition may then be included in the equationsn#33;#33;#61;n(n-2)#33;#33;,#92;quad 1#33;#33;#61;1,#92;quad 2#33;#33;#61;2.

词形变化

double factorials plural

词源

From double + factorial.
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