Pattern-Avoiding Fishburn Permutations and Ascent Sequences

TL;DR

This paper explores pattern-avoiding Fishburn permutations and ascent sequences, establishing bijections, enumerating specific classes, and deriving formulas involving Fibonacci numbers and powers of two.

Contribution

It proves a conjecture linking Fishburn permutations and ascent sequences, and provides new enumeration formulas for various pattern-avoiding classes.

Findings

Bijection between $F_n(3412)$ and $A_n(201)$

Enumeration of $F_n(123)$ with respect to inversion and maxima

Exact counts for certain pattern-avoiding classes involving Fibonacci numbers and powers of two

Abstract

A Fishburn permutation is a permutation which avoids the bivincular pattern $(231, \{1\}, \{1\})$, while an ascent sequence is a sequence of nonnegative integers in which each entry is less than or equal to one more than the number of ascents to its left. Fishburn permutations and ascent sequences are linked by a bijection $g$ of Bousquet-M\'elou, Claesson, Dukes, and Kitaev. We write $F_n(\sigma_1,\ldots,\sigma_k)$ to denote the set of Fishburn permutations of length $n$ which avoid each of $\sigma_1,\ldots,\sigma_k$ and we write $A_n(\alpha_1,\ldots,\alpha_k)$ to denote the set of ascent sequences which avoid each of $\alpha_1,\ldots,\alpha_k$. We settle a conjecture of Gil and Weiner by showing that $g$ restricts to a bijection between $F_n(3412)$ and $A_n(201)$. Building on work of Gil and Weiner, we use elementary techniques to enumerate $F_n(123)$ with respect to inversion number and number of left-to-right maxima, obtaining expressions in terms of $q$-binomial coefficients, and to enumerate $F_n(123,\sigma)$ for all $\sigma$. We use generating tree techniques to study the generating functions for $F_n(321, 1423)$, $F_n(321,3124)$, and $F_n(321,2143)$ with respect to inversion number and number of left-to-right maxima. We use these results to show $|F_n(321,1423)| = |F_n(321,3124)| = F_{n+2} - n - 1$, where $F_n$ is a Fibonacci number, and $|F_n(321,2143)| = 2^{n-1}$. We conclude with a variety of conjectures and open problems.