Permutations Avoiding a Simsun Pattern

Abstract

A permutation $\pi$ avoids the simsun pattern $\tau$ if $\pi$ avoids the consecutive pattern $\tau$ and the same condition applies to  the restriction of $\pi$ to any interval $[k].$ Permutations avoiding the simsun pattern $321$ are the usual simsun permutation introduced by Simion and Sundaram. Deutsch and Elizalde enumerated the set of simsun permutations that avoid in addition any set of patterns of length $3$ in the classical sense. In this paper we enumerate the set of permutations avoiding any other simsun pattern of length $3$ together with any set of classical patterns of length $3.$ The main tool in the proofs is a massive use of a bijection between permutations and increasing binary trees.