Vendredi 3 juin 2016 à 11h, salle 24-25/405
Given a colouring of a graph, a Kempe change is the operation of picking a maximal bichromatic subgraph and switching the two colours in it. Two colourings are Kempe equivalent if they can be obtained from each other through a series of Kempe changes. Kempe changes were first introduced in a failed attempt to prove the Four Colour Theorem, but they proved to be a powerful tool for other colouring problems. They are also relevant for more applied questions, most notably in theoretical physics. Consider a graph with no vertex of degree more than some integer D. In 2007, Mohar conjectured that all its D-colourings are Kempe-equivalent. Feghali, Johnson and Paulusma proved in 2015 that this is true for D=3, with the exception of one single graph which disproves the conjecture in its generality. We settle the remaining cases by proving the conjecture holds for every integer D at least 4. This is a joint work with Nicolas Bousquet (LIRIS, Ecole Centrale Lyon, France), Carl Feghali (Durham University, UK) and Matthew Johnson (Durham University, UK).