A Reconfigurations Analogue of Brooks' Theorem and Its Consequences

Abstract

AbstractLet G be a simple undirected connected graph on n vertices with maximum degree Δ. Brooks' Theorem states that G has a proper Δ‐coloring unless G is a complete graph, or a cycle with an odd number of vertices. To recolor G is to obtain a new proper coloring by changing the color of one vertex. We show an analogue of Brooks' Theorem by proving that from any k‐coloring, , a Δ‐coloring of G can be obtained by a sequence of recolorings using only the original k colors unless G is a complete graph or a cycle with an odd number of vertices, or , G is Δ‐regular and, for each vertex v in G, no two neighbors of v are colored alike. We use this result to study the reconfiguration graph of the k‐colorings of G. The vertex set of is the set of all possible k‐colorings of G and two colorings are adjacent if they differ on exactly one vertex. We prove that for , consists of isolated vertices and at most one further component that has diameter . This result enables us to complete both a structural and an algorithmic characterization for reconfigurations of colorings of graphs of bounded maximum degree.