The centralizer of two numbers under the natural action of $S_k$ on [k], the maximal parabolic subgroup of $S_k$, and generalized patterns

Abstract

A natural generalization of single pattern avoidance is subset avoidance. A complete study of subset avoidance for the case k=3 is carried out in [SS]. For k>3 situation becomes more complicated, as the number of possible cases grows rapidly. Recently, several authors have considered the case of general k when T has some nice algebraic properties. Barcucci, Del Lungo, Pergola, and Pinzani in [BDPP) treated the case when $T=T_1$ is the centralizer of k-1 and k under the natural action of $S_k$ on [k]. Mansour and Vainshtein in [MVp] treated the case when $T=T_2$ is maximal parabolic group of $S_k$. Recently, Babson and Steingrimsson (see [BS]) introduced generalized permutations patterns that allow the requirement that two adjacent letters in a pattern must be adjacent in the permutation. In this paper we present an analogue with generalization for the case $T_1$ and for the case $T_2$ by using generalized patterns instead of classical patterns.