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Enumerations Of Specific Permutation Classes
In the study of permutation patterns, there has been considerable interest in enumerating specific permutation classes, especially those with relatively few basis elements. This area of study has turned up unexpected instances of Wilf equivalence, where two seemingly-unrelated permutation classes have the same number of permutations of each length. Classes avoiding one pattern of length 3 There are two symmetry classes and a single Wilf class for single permutations of length three. Classes avoiding one pattern of length 4 There are seven symmetry classes and three Wilf classes for single permutations of length four. No non-recursive formula counting 1324-avoiding permutations is known. A recursive formula was given by . A more efficient algorithm using functional equations was given by , which was enhanced by , and then further enhanced by who give the first 50 terms of the enumeration. currently have the best rigorously established lower and upper bounds for the grow ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Permutation Pattern
In combinatorial mathematics and theoretical computer science, a (classical) permutation pattern is a sub-permutation of a longer permutation. Any permutation may be written in one-line notation as a sequence of entries representing the result of applying the permutation to the sequence 123...; for instance the sequence 213 represents the permutation on three elements that swaps elements 1 and 2. If π and σ are two permutations represented in this way (these variable names are standard for permutations and are unrelated to the number pi), then π is said to ''contain'' σ as a ''pattern'' if some subsequence of the entries of π has the same relative order as all of the entries of σ. For instance, permutation π contains the pattern 213 whenever π has three entries ''x'', ''y'', and ''z'' that appear within π in the order ''x''...''y''...''z'' but whose values are ordered as ''y'' < ''x'' < ''z'', the same as the ordering of the values in the permutation 2 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
