On stable patterns and properties on permutations of multisets

TL;DR

This paper investigates stable patterns in permutations of multisets with symmetric generating functions, characterizing stable classical patterns and analyzing stability of monotone and other consecutive patterns.

Contribution

It provides a complete characterization of stable classical patterns and establishes stability results for monotone consecutive patterns, introducing new combinatorial interpretations.

Findings

Only patterns of length one or two are stable among classical patterns.

All monotone consecutive patterns are stable.

A large class of unstable consecutive patterns identified.

Abstract

In this paper, we study properties and patterns on permutations of multisets whose multivariate generating functions are symmetric. We interpret this phenomenon through the lens of group actions and define such a property or pattern as stable. We provide a complete characterization of stable classical patterns, showing that the only such patterns are those of length one or two. For consecutive patterns, we establish the stability of all monotone patterns and also identify a large class of unstable patterns. We conjecture that monotone patterns are the only stable consecutive patterns. All stability results in this paper are proven via explicit bijections, which provide new combinatorial interpretations of the symmetry of the generating functions. As an application, we use stability to derive recurrence relations for the ascent distribution on permutations of multisets, resulting in a generalization of Eulerian numbers.