Wilf equivalence

In the study of permutations and permutation patterns, Wilf equivalence is an equivalence relation on permutation classes. Two permutation classes are Wilf equivalent when they have the same numbers of permutations of each possible length, or equivalently if they have the same generating functions. The equivalence classes for Wilf equivalence are called Wilf classes; they are the combinatorial classes of permutation classes. The counting functions and Wilf equivalences among many specific permutation classes are known.

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