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Chord theorems on graphs (revised and extended)

Mohammad Javaheri
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Abstract

The Horizontal Chord Theorem states that if a continuous curve connects points $A$ and $B$ in the plane, then for every positive integer $k$ there are points $C$ and $D$ on the curve with $\overrightarrow{AB}=k\,\overrightarrow{CD}$. We reformulate the one-dimensional chord problem on finite metric graphs: the \emph{chord set} $\cH(G)$ consists of those $r$ such that every function of mean zero on $G$ integrates to zero over some closed connected subset of $G$ of measure $r$. We prove that the space of connected subsets of fixed measure is compact and path-connected, that $\cH(G)$ always contains the closure of a combinatorially defined \emph{partition set} $\cP(G)$, and that the two sets agree at every rational number; as a consequence, membership of a rational number in $\cH(G)$ is decided by a finite linear program, and the conjecture $\cH(G)=\overline{\cP(G)}$ reduces to a statement about irrational points. We give a covering lemma that yields $\cH(G)=[0,|E|]$ for Eulerian graphs, $[0,1]\subseteq\cH(G)$ when $G$ has no vertex of degree one, and $[0,g(G)]\subseteq\cH(G)$ when $G$ is bridgeless with girth $g(G)$. Conversely, a bridge forces a gap in $\cH(G)$ below $|E|$. Computations on small graphs support the conjecture that $\cH(G)=[0,|E|]$ exactly when $G$ is bridgeless.

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Mohammad Javaheri. 2026-10-25. Chord theorems on graphs (revised and extended). https://arxiv.org/abs/math/0702805

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