On Pattern Avoiding Alternating Permutations

Abstract

An alternating permutation of length $n$ is a permutation $π=π_1 π_2 ... π_n$ such that $π_1 < π_2 > π_3 < π_4 > ...$. Let $A_n$ denote set of alternating permutations of ${1,2,..., n}$, and let $A_n(σ)$ be set of alternating permutations in $A_n$ that avoid a pattern $σ$. Recently, Lewis used generating trees to enumerate $A_{2n}(1234)$, $A_{2n}(2143)$ and $A_{2n+1}(2143)$, and he posed several conjectures on the Wilf-equivalence of alternating permutations avoiding certain patterns. Some of these conjectures have been proved by Bóna, Xu and Yan. In this paper, we prove the two relations $|A_{2n+1}(1243)|=|A_{2n+1}(2143)|$ and $|A_{2n}(4312)|=|A_{2n}(1234)|$ as conjectured by Lewis.

21 pages, 2 figures