Counting tight Hamilton cycles in Dirac hypergraphs

Abstract

Suppose $G$ is a $k$-uniform hypergraph on $n$ vertices such that every $(k-1)$-subset $S$ of $V(G)$ belongs to at least $\hat{I}´n$ edges, where $\hat{I}´>1/2$. Let $\hat{I}¨(G)$ denote the number of tight Hamilton cycles in $G$, that is, cyclic orderings of $V(G)$ in which every $k$ consecutive vertices form an edge. We prove that $\log \hat{I}¨(G)\ge kh(G)-n\log \left(\genfrac{}{}{0ex}{}{n}{k-1}\right)+n\log n-n\log e-o(n)$, where $h(G)$ is the hypergraph entropy of $G$, defined via perfect fractional matchings. This bound is tight, for example, for all (nearly) regular hypergraphs, in particular for the binomial random hypergraph. It also implies a conjecture by Ferber, Hardiman and Mond, stating that $\hat{I}¨(G)\ge (\hat{I}´-o(1){)}^{n}n!$.

23 pages + 9 pages appendix for general l-cycles