Heaps and Two Exponential Structures

TL;DR

This paper explores the combinatorial structures of certain posets related to set partitions and introduces new connections to Euler numbers, non-ambiguous trees, and permutation pairs, providing novel proofs and bijections.

Contribution

It proves that specific enumeration sequences for these posets correspond to well-known combinatorial numbers and constructs new bijections, extending existing theorems and solving open problems.

Findings

r_n(Π_n^{(r)}) equals generalized Euler number E_{rn-1}

r_n(Q_n^{(2)}) counts complete non-ambiguous trees

Establishes a bijection between non-ambiguous forests and permutation pairs with no common rise

Abstract

Take ${\sf Q}=({\sf Q}_1,{\sf Q}_2,\ldots)$ to be an exponential structure and $M(n)$ to be the number of minimal elements of ${\sf Q}_n$ where $M(0)=1$. Then a sequence of numbers $\{r_n({\sf Q}_n)\}_{n\ge 1}$ is defined by the equation \begin{eqnarray*} \sum_{n\ge 1}r_n({\sf Q}_n)\frac{z^n}{n!\,M(n)}=-\log(\sum_{n\ge 0}(-1)^n\frac{z^n}{n!\,M(n)}). \end{eqnarray*} Let $\bar{{\sf Q}}_n$ denote the poset ${\sf Q}_n$ with a $\hat{0}$ adjoined and let $\hat{1}$ denote the unique maximal element in the poset ${\sf Q}_n$. Furthermore, let $\mu_{{\sf Q}_n}$ be the M\"{o}bius function on the poset $\bar{{\sf Q}}_n$. Stanley proved that $r_n({\sf Q}_n)=(-1)^n\mu_{{\sf Q}_n}(\hat{0},\hat{1})$. This implies that the numbers $r_n({\sf Q}_n)$ are integers. In this paper, we study the cases ${\sf Q}_n=\Pi_n^{(r)}$ and ${\sf Q}_n={\sf Q}_n^{(r)}$ where $\Pi_n^{(r)}$ and ${\sf Q}_n^{(r)}$ are posets, respectively, of set partitions of $[rn]$ whose block sizes are divisible by $r$ and of $r$-partitions of $[n]$. In both cases we prove that $r_n(\Pi_n^{(r)})$ and $r_n({\sf Q}_n^{(r)})$ enumerate the pyramids by applying the Cartier-Foata monoid identity and further prove that $r_n(\Pi_n^{(r)})$ is the generalized Euler number $E_{rn-1}$ and that $r_n({\sf Q}_n^{(2)})$ is the number of complete non-ambiguous trees of size $2n-1$ by bijections. This gives a new proof of Welker's theorem that $r_n(\Pi_n^{(r)})=E_{rn-1}$ and implies the construction of $r$-dimensional complete non-ambiguous trees. As a bonus of applying the theory of heaps, we establish a bijection between the set of complete non-ambiguous forests and the set of pairs of permutations with no common rise. This answers an open question raised by Aval {\it et al.}.