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All Journal Electronic Journal of Graph Theory and Applications (EJGTA)
Hans Cuypers
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Electrical & Electronics Engineering

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Whitney’s theorem for line graphs of multi-graphs Hans Cuypers
Electronic Journal of Graph Theory and Applications (EJGTA) Vol 14, No 2 (2026): Electronic Journal of Graph Theory and Applications
Publisher : GTA Research Group, Univ. Newcastle, Indonesian Combinatorics Society and ITB

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/ejgta.2026.14.2.14

Abstract

Whitney’s Theorem states that every connected graph, different from K3 or K1,3, is uniquely determined by its line graph. The graphs K1,3 and K3 have isomorphic line graphs. We provide an analogue of Whitney’s Theorem for line graphs of multi-graphs. There are two obvious choices for the definition of a line graph of a multi-graph. A 1-line graph of a multi-graph is the graph with as vertices the edges of the multi-graph, and two edges adjacent if and only if there is a unique vertex which is on both edges. The ≥ 1-line graph of a multi-graph is the graph on the edges of the multi-graph, where two edges are adjacent if and only if there is at least one vertex which is on both edges. We extend Whitney’s Theorem to both classes of line graphs of multi-graphs, and show that most multi-graphs are uniquely determined by their line graph. Moreover, we present an algorithm to determine for a given graph Γ, if possible, a multi-graph with Γ as line graph.
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