On Weighted Star--Convex Graphs

TL;DR

This paper explores the concepts of geometric and sequential convexity in graphs, characterizing star-convex graphs and embedding convex sequences into star-convex structures like spider graphs.

Contribution

It establishes a characterization of star-convex graphs via trees containing all leaves and demonstrates embedding convex sequences into star-convex graphs.

Findings

A graph is star-convex iff it contains a star-convex tree with all leaves.

Convex sequences can be embedded into spider graphs to form star-convex structures.

Characterization links geometric and sequential convexity in graph theory.

Abstract

The primary objective of this paper is to investigate the notions of geometric and sequential convexity within a graph-theoretic framework, with the aim of examining various structural properties and exploring the connection between these two branches of mathematics. A simple connected vertex-weighted graph $G(V,E)$ with a non-empty set of leaf vertices is said to be star-convex if there exists at least one node $u\in V(G)$ such that, for every chosen leaf vertex $v$, there is a monotone path (either increasing or decreasing) connecting $v$ to $u$. One of the main results states that a graph $G$ is star-convex if and only if there exists a tree $T\subseteq G$ that contains all leaf vertices and is itself star-convex. On the other hand, a sequence $\big(u_n\big)_{n=0}^{\infty}$ is said to be convex if it satisfies the following inequality $$ 2u_{i}\leq u_{i-1}+u_{i+1}\qquad \mbox{for all}\quad i\in \mathbb{N}. $$ We demonstrate that, under minimal assumptions, a class of convex sequences can be embedded into a spider graph so as to make it star-convex.