Enumeration and randomized constructions of hypertrees

TL;DR

This paper advances the understanding of the enumeration of $d$-hypertrees by providing improved lower bounds and analyzing a random model that results in complexes with negligible $d$-dimensional homology.

Contribution

It offers a significant improvement in the lower bounds for counting unweighted $d$-hypertrees and studies a random construction model with minimal $d$-homology.

Findings

Improved lower bounds for the number of $d$-hypertrees.

Random $1$-out model produces complexes with negligible $d$-homology.

Extends enumeration results in higher-dimensional combinatorial topology.

Abstract

Over thirty years ago, Kalai proved a beautiful $d$-dimensional analog of Cayley's formula for the number of $n$-vertex trees. He enumerated $d$-dimensional hypertrees weighted by the squared size of their $(d-1)$-dimensional homology group. This, however, does not answer the more basic problem of unweighted enumeration of $d$-hypertrees, which is our concern here. Our main result, Theorem 1.4, significantly improves the lower bound for the number of $d$-hypertrees. In addition, we study a random $1$-out model of $d$-complexes where every $(d-1)$-dimensional face selects a random $d$-face containing it, and show it has a negligible $d$-dimensional homology.