Powers of permutations that avoid chains of patterns

Abstract

In a recent paper, Bona and Smith define the notion of \textit{strong avoidance}, in which a permutation and its square both avoid a given pattern. In this paper, we generalize this idea to what we call \textit{chain avoidance}. We say that a permutation avoids a chain of patterns $({\tau }_1:{\tau }_2:\cdots :{\tau }_k)$ if the $i$-th power of the permutation avoids the pattern ${\tau }_i$. We enumerate the set of permutations $\pi$ which avoid the chain $(213,312:\tau )$, i.e.,~unimodal permutations whose square avoids $\tau$, for $\tau \in {\S }_3$ and use this to find a lower bound on the number of permutations that avoid the chain $(312:\tau )$ for $\tau \in {\S }_3$. We finish the paper by discussing permutations that avoid longer chains.