More results on stack-sorting for set partitions

Abstract

Let a sock be an element of an ordered finite alphabet A and a sequence of these elements be a sock sequence. In 2023, Xia introduced a deterministic version of Defant and Kravitz's stack-sorting map by defining the $\phi_{\sigma}$ and $\phi_{\overline{\sigma}}$ pattern-avoidance stack-sorting maps for sock sequences. Xia showed that the $\phi_{aba}$ map is the only one that eventually sorts all set partitions; in this paper, we prove deeper results regarding $\phi_{aba}$ and $\phi_{\overline{aba}}$ as a natural next step. We newly define two algorithms with time complexity $O(n^3)$ that determine if any given sock sequence is in the image of $\phi_{aba}$ or $\phi_{\overline{aba}}$ respectively. We also show that the maximum number of preimages that a sock sequence of length $n$ has grows at least exponentially under both the $\phi_{aba}$ and $\phi_{\overline{aba}}$ maps. Additionally, we prove results regarding fertility numbers (introduced by Defant) in the context of set partitions and multiple-pattern-avoiding stacks.