Wilf equivalence relations for consecutive patterns

Abstract

Two permutations $\pi$ and $\tau$ are c-Wilf equivalent if, for each $n$, the number of permutations in $S_n$ avoiding $\pi$ as a consecutive pattern (i.e., in adjacent positions) is the same as the number of those avoiding $\tau$. In addition, $\pi$ and $\tau$ are strongly c-Wilf equivalent if, for each $n$ and $k$, the number of permutations in $S_n$ containing $k$ occurrences of $\pi$ as a consecutive pattern is the same as for $\tau$. In this paper we introduce a third, more restrictive equivalence relation, defining $\pi$ and $\tau$ to be super-strongly c-Wilf equivalent if the above condition holds for any set of prescribed positions for the $k$ occurrences. We show that, when restricted to non-overlapping permutations, these three equivalence relations coincide. We also give a necessary condition for two permutations to be strongly c-Wilf equivalent. Specifically, we show that if $\pi,\tau$ in $S_m$ are strongly c-Wilf equivalent, then $|\pi_m-\pi_1|=|\tau_m-\tau_1|$. In the special case of non-overlapping permutations $\pi$ and $\tau$, this proves a weaker version of a conjecture of the second author stating that $\pi$ and $\tau$ are c-Wilf equivalent if and only if $\pi_1=\tau_1$ and $\pi_m=\tau_m$, up to trivial symmetries. Finally, we strengthen a recent result of Nakamura and Khoroshkin-Shapiro giving sufficient conditions for strong c-Wilf equivalence.