CDAM: Computational, Discrete and Applicable Mathematics@LSE

CDAM Research Report, LSE-CDAM-2007-31

December 2007

Finding Paths Between 3-Colourings

Luis Cereceda, Jan van den Heuvel, and Matthew Johnson

Given a 3-colourable graph G and two proper vertex 3-colourings α and β of G, consider the following question: is it possible to transform α into β by recolouring vertices of G one at a time, making sure that all intermediate colourings are proper 3-colourings?
We prove that this question is answerable in polynomial time. We do so by characterising the instances G,α,β where the transformation is possible; the proof of this characterisation is via an algorithm that either finds a sequence of recolourings between α and β, or exhibits a structure which proves that no such sequence exists. In the case that a sequence of recolourings does exist, the algorithm uses O(|V(G)|²) recolouring steps and in many cases returns a shortest sequence of recolourings. We also exhibit a class of instances G,α,β that require Ω(|V(G)|²) recolouring steps.

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		Phone: +44(0)-20-7955 7494. Fax: +44(0)-20-7955 6877. Email: info@maths.lse.ac.uk