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In mathematics, the double factorial of a number n, denoted by n‼, is the product of all the positive integers up to n that have the same parity (odd or even) as n. [1] That is, n ! ! = ∏ k = 0 ⌈ n 2 ⌉ − 1 ( n − 2 k ) = n ( n − 2 ) ( n − 4 ) ⋯ . {\displaystyle n!!=\prod _{k=0}^{\left\lceil {\frac {n}{2}}\right\rceil -1}(n-2k ...
The factorial of is , or in symbols, ! =. There are several motivations for this definition: For n = 0 {\displaystyle n=0} , the definition of n ! {\displaystyle n!} as a product involves the product of no numbers at all, and so is an example of the broader convention that the empty product , a product of no factors, is equal to the ...
In this article, the symbol () is used to represent the falling factorial, and the symbol () is used for the rising factorial. These conventions are used in combinatorics , [ 4 ] although Knuth 's underline and overline notations x n _ {\displaystyle x^{\underline {n}}} and x n ¯ {\displaystyle x^{\overline {n}}} are increasingly popular.
1. Factorial: if n is a positive integer, n! is the product of the first n positive integers, and is read as "n factorial". 2. Double factorial: if n is a positive integer, n!! is the product of all positive integers up to n with the same parity as n, and is read as "the double factorial of n". 3.
The following table lists many specialized symbols commonly used in modern mathematics, ordered by their introduction date. The table can also be ordered alphabetically by clicking on the relevant header title.
The Miscellaneous Mathematical Symbols-B block (U+2980–U+29FF) contains miscellaneous mathematical symbols, including brackets, angles, and circle symbols. Miscellaneous Mathematical Symbols-B [1] Official Unicode Consortium code chart (PDF)
In each term of the first sum, gives the number of matched pairs, the binomial coefficient counts the number of ways of choosing the elements to be matched, and the double factorial ()!! = ()!! is the product of the odd integers up to its argument and counts the number of ways of completely matching the 2k selected elements.
Note: The empty set symbol ∅ looks similar, but is unrelated to the Greek letter. or represents: the golden ratio 1.618... in mathematics, art, and architecture; Euler's totient function in number theory; the argument of a complex number in mathematics; the value of a plane angle in physics and mathematics