On Refinements of Wilf-Equivalence for Involutions

Abstract

Let $\mathcal{S}_n(\pi)$ (resp. $\mathcal{I}_n(\pi)$ and $\mathcal{AI}_n(\pi)$) denote the set of permutations (resp. involutions and alternating involutions) of length $n$ which avoid the permutation pattern $\pi$. For $k,m\geq 1$, Backelin-West-Xin proved that $|\mathcal{S}_n(12\cdots k\tau)|= |\mathcal{S}_n(k\cdots 21\tau)|$ by establishing a bijection between these two sets, where $\tau = \tau_1\tau_2\cdots \tau_m$ is an arbitrary permutation of $k+1,k+2,\ldots,k+m$. The result has been extended to involutions by Bousquet-M\'elou and Steingr\'imsson and to alternating permutations by the first author. In this paper, we shall establish a peak set preserving bijection between $\mathcal{I}_n(123\tau)$ and $\mathcal{I}_n(321\tau)$ via transversals, matchings, oscillating tableaux and pairs of noncrossing Dyck paths as intermediate structures. Our result is a refinement of the result of Bousquet-M\'elou and Steingr\'imsson for the case when $k=3$. As an application, we show bijectively that $|\mathcal{AI}_n(123\tau)| = |\mathcal{AI}_n(321\tau)|$, confirming a recent conjecture of Barnabei-Bonetti-Castronuovo-Silimbani. Furthmore, some conjectured equalities posed by Barnabei-Bonetti-Castronuovo-Silimbani concerning pattern avoiding alternating involutions are also proved.