Hyperbolicity in the corona and join of graphs

TL;DR

This paper investigates hyperbolicity in specific graph operations, characterizing when graph joins and coronas are hyperbolic and providing formulas for their hyperbolicity constants.

Contribution

It introduces new characterizations of hyperbolic properties for graph join and corona operations, including explicit formulas for their hyperbolicity constants.

Findings

Graph join $G_12$ is always hyperbolic.

Corona $G_1 riangle G_2$ is hyperbolic iff $G_1$ is hyperbolic.

Provided explicit formulas for hyperbolicity constants of join and corona graphs.

Abstract

If X is a geodesic metric space and $x_1,x_2,x_3\in X$, a {\it geodesic triangle} $T=\{x_1,x_2,x_3\}$ is the union of the three geodesics $[x_1x_2]$, $[x_2x_3]$ and $[x_3x_1]$ in $X$. The space $X$ is $\delta$-\emph{hyperbolic} $($in the Gromov sense$)$ if any side of $T$ is contained in a $\delta$-neighborhood of the union of the two other sides, for every geodesic triangle $T$ in $X$. If $X$ is hyperbolic, we denote by $\delta(X)$ the sharp hyperbolicity constant of $X$, i.e. $\delta(X)=\inf\{\delta\ge 0: \, X \, \text{ is $\delta$-hyperbolic}\,\}\,.$ Some previous works characterize the hyperbolic product graphs (for the Cartesian product, strong product and lexicographic product) in terms of properties of the factor graphs. In this paper we characterize the hyperbolic product graphs for graph join $G_1\uplus G_2$ and the corona $G_1\diamond G_2$: $G_1\uplus G_2$ is always hyperbolic, and $G_1\diamond G_2$ is hyperbolic if and only if $G_1$ is hyperbolic. Furthermore, we obtain simple formulae for the hyperbolicity constant of the graph join $G_1\uplus G_2$ and the corona $G_1\diamond G_2$.