On the number of forests and connected spanning subgraphs

Márton Borbényi, Péter Csikvári, Haoran Luo

Published 2020-05-26Version 1

Let $F(G)$ be the number of forests of a graph $G$. Similarly let $C(G)$ be the number of connected spanning subgraphs of a connected graph $G$. We bound $F(G)$ and $C(G)$ for regular graphs and for graphs with fixed average degree. Among many other things we study $f_d=\sup_{G\in \mathcal{G}_d}F(G)^{1/v(G)}$, where $\mathcal{G}_d$ is the family of $d$--regular graphs, and $v(G)$ denotes the number of vertices of a graph $G$. We show that $f_3=2^{3/2}$, and if $(G_n)_n$ is a sequence of $3$--regular graphs with length of the shortest cycle tending to infinity, then $\lim_{n\to \infty}F(G_n)^{1/v(G_n)}=2^{3/2}$. We also improve on the previous best bounds on $f_d$ for $4\leq d\leq 9$. We give various applications of our inequalities to grids.