Two patterns T_1 and T_2 belong to the same Wilf class if left bracketing bar S_n(T_1) right bracketing bar = left bracketing bar S_n(T_2) right bracketing bar for all n, where S_n(T) denotes the set of permutations on {1, ..., n} that avoid the pattern T. Two sets having the same Wilf class are said to be Wilf equivalent.

avoided pattern | permutation pattern | Wilf equivalent