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The birthday problem has been generalized to consider an arbitrary number of types. [20] In the simplest extension there are two types of people, say m men and n women, and the problem becomes characterizing the probability of a shared birthday between at least one man and one woman. (Shared birthdays between two men or two women do not count.)
Enumerations of specific permutation classes; Factorial. Falling factorial; Permutation matrix. Generalized permutation matrix; Inversion (discrete mathematics) Major index; Ménage problem; Permutation graph; Permutation pattern; Permutation polynomial; Permutohedron; Rencontres numbers; Robinson–Schensted correspondence; Sum of permutations ...
Combinations and permutations in the mathematical sense are described in several articles. Described together, in-depth: Twelvefold way; Explained separately in a more accessible way: Combination; Permutation; For meanings outside of mathematics, please see both words’ disambiguation pages: Combination (disambiguation) Permutation ...
The formula counting all functions N → X is not useful here, because the number of them grouped together by permutations of N varies from one function to another. Rather, as explained under combinations , the number of n -multicombinations from a set with x elements can be seen to be the same as the number of n -combinations from a set with x ...
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A different rule for multiplying permutations comes from writing the argument to the left of the function, so that the leftmost permutation acts first. [ 30 ] [ 31 ] [ 32 ] In this notation, the permutation is often written as an exponent, so σ acting on x is written x σ ; then the product is defined by x σ ⋅ τ = ( x σ ) τ ...
An involution is a permutation σ so that σ 2 = 1 under permutation composition. It follows that σ may only contain cycles of length one or two, i.e. the exponential generating function g(z) of these permutations is [1] = (+).
These identities may be derived by enumerating permutations directly. For example, a permutation of n elements with n − 3 cycles must have one of the following forms: n − 6 fixed points and three two-cycles; n − 5 fixed points, a three-cycle and a two-cycle, or; n − 4 fixed points and a four-cycle.