Counting permutations avoiding two flat partially ordered patterns

TL;DR

This paper studies permutations avoiding two flat partially ordered patterns, establishing connections to k-Fibonacci numbers, providing bijections, and deriving generating functions, including for separable permutations with length up to 5.

Contribution

It introduces the study of permutations avoiding two flat POPs, linking to k-Fibonacci numbers and providing explicit generating functions and enumerative results.

Findings

Permutations avoiding two flat POPs relate to k-Fibonacci numbers.

A bijection is established between such permutations and restricted permutations.

The generating function for these permutations with both patterns of length 5 is rational, with many monomials.

Abstract

Partially ordered patterns (POPs) play an important role in the study of permutation patterns, providing a convenient framework for describing large families of classical patterns. The problem of enumerating permutations that avoid POPs has therefore attracted considerable attention in the literature. In particular, Gao and Kitaev resolved many counting problems for POP-avoiding permutations of lengths 4 and 5, linking the enumeration to a wide range of other combinatorial objects. Motivated by their work, we initiate the study of permutations that simultaneously avoid two POPs belonging to the class of flat POPs. We establish a connection between permutations avoiding such POPs and the $k$-Fibonacci numbers. Moreover, we provide a bijection between permutations avoiding these POPs and certain restricted permutations, which allows us to use the method developed by Balti\'{c} to derive the generating function for permutations avoiding these POPs. Finally, we obtain enumerative results for separable permutations avoiding these two POPs, of lengths up to 5, with respect to six statistics, thereby extending the results of Gao et al. on the avoidance of a single flat POP in separable permutations. Notably, when both patterns are of length 5, the respective generating function is a rational function, with the sum in the numerator (resp., denominator) containing 293 (resp., 17) monomials.