Deterministic stack-sorting for set partitions

Abstract

A sock sequence is a sequence of elements, which we will refer to as socks, from a finite alphabet. A sock sequence is sorted if all occurrences of a sock appear consecutively. We define equivalence classes of sock sequences called sock patterns, which are in bijection with set partitions. The notion of stack-sorting for set partitions was originally introduced by Defant and Kravitz. In this paper, we define a new deterministic stack-sorting map $ϕ_σ$ for sock sequences that uses a $σ$-avoiding stack, where pattern containment need not be consecutive. When $σ= aba$, we show that our stack-sorting map sorts any sock sequence with $n$ distinct socks in at most $n$ iterations, and that this bound is tight for $n \geq 3$. We obtain a fine-grained enumeration of the number of sock patterns of length $n$ on $r$ distinct socks that are $1$-stack-sortable under $ϕ_{aba}$, and we also obtain asymptotics for the number of sock patterns of length $n$ that are $1$-stack-sortable under $ϕ_{aba}$. Finally, we show that for all unsorted sock patterns $σ\neq a\cdots a b a \cdots a$, the map $ϕ_σ$ cannot eventually sort all sock sequences on any multiset $M$ unless every sock sequence on $M$ is already sorted.