The permutation class Av(4213,2143)

TL;DR

This paper characterizes the structure of permutations avoiding patterns 4213 and 2143, derives their generating function, and analyzes properties of a typical large permutation in this class.

Contribution

It provides a detailed structural description of the permutation class Av(4213,2143) and computes its generating function, advancing understanding of pattern-avoiding permutations.

Findings

Permutations in the class are skew sums of plane trees with added increasing sequences.

The generating function for these permutations is explicitly derived.

A typical large permutation in the class is more likely to also avoid 2413.

Abstract

We determine the structure of permutations avoiding the patterns 4213 and 2143. Each such permutation consists of the skew sum of a sequence of plane trees, together with an increasing sequence of points above and an increasing sequence of points to its left. We use this characterisation to establish the generating function enumerating these permutations. We also investigate the properties of a typical large permutation in the class and prove that if a large permutation that avoids 4213 and 2143 is chosen uniformly at random, then it is more likely than not to avoid 2413 as well.