Elsevier

European Journal of Combinatorics

Volume 76, February 2019, Pages 159-174
European Journal of Combinatorics

Hadwiger’s conjecture for squares of 2-trees

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Abstract

Hadwiger’s conjecture asserts that any graph contains a clique minor with order no less than the chromatic number of the graph. We prove that this well-known conjecture is true for all graphs if and only if it is true for squares of split graphs. This observation implies that Hadwiger’s conjecture for squares of chordal graphs is as difficult as the general case, since chordal graphs are a superclass of split graphs. Then we consider 2-trees which are a subclass of each of planar graphs, 2-degenerate graphs and chordal graphs. We prove that Hadwiger’s conjecture is true for squares of 2-trees. We achieve this by proving the following stronger result: for any 2-tree T, its square T2 has a clique minor of order χ(T2) for which each branch set induces a path, where χ(T2) is the chromatic number of T2.

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1

Part of the work was done when this author was visiting Max Planck Institute for Informatics, Saarbruecken, Germany supported by Alexander von Humboldt Fellowship, Germany.

2

Research was supported by ARC Discovery Project, Australia DP120101081.