Efficient recurrence for the enumeration of permutations with fixed pinnacle set

TL;DR

This paper introduces an efficient recurrence relation to compute the number of permutations with a fixed pinnacle set, providing new formulas and algorithms that improve computational efficiency and deepen combinatorial understanding.

Contribution

It presents a new recurrence for counting permutations with a given pinnacle set and offers an alternative, combinatorially flavored form for a related weighted sum, along with efficient algorithms for admissible orderings.

Findings

Recurrence with $O(k^4 + k\log n)$ complexity for counting permutations with fixed pinnacle sets.

A new combinatorial formula for the weighted sum $q_n(P)$ of permutation counts.

Efficient algorithms for counting admissible orderings of pinnacle sets.

Abstract

Initiated by Davis, Nelson, Petersen and Tenner (2018), the enumerative study of pinnacle sets of permutations has attracted a fair amount of attention recently. In this article, we provide a recurrence that can be used to compute efficiently the number $|\mathfrak{S}_n(P)|$ of permutations of size $n$ with a given pinnacle set $P$, with arithmetic complexity $O(k^4 + k\log n)$ for $P$ of size $k$. A symbolic expression can also be computed in this way for pinnacle sets of fixed size. A weighted sum $q_n(P)$ of $|\mathfrak{S}_n(P)|$ proposed in Davis, Nelson, Petersen and Tenner (2018) seems to have a simple form, and a conjectural form is given recently by Flaque, Novelli and Thibon (2021+). We settle the problem by providing and proving an alternative form of $q_n(P)$, which has a strong combinatorial flavor. We also study admissible orderings of a given pinnacle set, first considered by Rusu (2020) and characterized by Rusu and Tenner (2021), and we give an efficient algorithm for their counting.