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Steinberg’s conjecture is false

Conjecture   Every planar graph without 4-cycles and 5-cycles is 3-colourable.


In a paper just uploaded to the arXivVincent Cohen-Addad, Michael Hebdige, Daniel Kral, Zhentao Li and Esteban Salgado show the construction of a graph with no cycles of length 4 or 5 which isn’t 3-colourable: it isn’t possible to assign colours to its vertices so that no pair of adjacent vertices have the same colour, using only three different colours. This is a counterexample to a conjecture of Richard Steinberg from 1976.

The problem was listed in the Open Problem Garden as of “outstanding” importance.

Read the paper: Steinberg’s conjecture is false

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