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تسجيل مستخدم جديدWarmth and connectivity of neighborhood complexes of graphs
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In this paper we study a pair of numerical parameters associated to a graph $G$. One the one hand, one can construct $text{Hom}(K_2, G)$, a space of homomorphisms from a edge $K_2$ into $G$ and study its (topological) connectivity. This approach dates back to the neighborhood complexes introduced by Lovasz in his proof of the Kneser conjecture. In another direction Brightwell and Winkler introduced a graph parameter called the warmth $zeta(G)$ of a graph $G$, based on asymptotic behavior of $d$-branching walks in $G$ and inspired by constructions in statistical physics. Both the warmth of $G$ and the connectivity of $text{Hom}(K_2,G)$ provide lower bounds on the chromatic number of $G$. Here we seek to relate these two constructions, and in particular we provide evidence for the conjecture that the warmth of a graph $G$ is always less than three plus the connectivity of $text{Hom}(K_2, G)$. We succeed in establishing a first nontrivial case of the conjecture, by showing that $zeta(G) leq 3$ if $text{Hom}(K_2,G)$ has an infinite first homology group. We also calculate warmth for a family of `twisted toroidal graphs that are important extremal examples in the context of $text{Hom}$ complexes. Finally we show that $zeta(G) leq n-1$ if a graph $G$ does not have the complete bipartite graph $K_{a,b}$ for $a+b=n$. This provides an analogue for a similar result in the context of $text{Hom}$ complexes.
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