Colourings of cubic graphs inducing isomorphic monochromatic subgraphs

A k–bisection of a bridgeless cubic graph G is a 2–colouring of its vertex set such that the colour classes have the same cardinality and all connected components in the two subgraphs induced by the colour classes (monochromatic components in what follows) have order at most k. Ban and Linial conjectured that every bridgeless cubic graph admits a 2–bisection except for the Petersen graph. A similar problem for the edge set of cubic graphs has been studied: Wormald conjectured that every cubic graph G with |E(G)| ≡ 0 mod 2 has a 2–edge colouring such that the two monochromatic subgraphs are isomorphic linear forests (i.e. a forest whose components are paths). Finally, Ando conjectured that every cubic graph admits a bisection such that the two induced monochromatic subgraphs are isomorphic. In this paper, we give a detailed insight into the conjectures of Ban–Linial and Wormald and provide evidence of a strong relation of both of them with Ando’s conjecture. Furthermore, we also give computational and theoretical evidence in their support. As a result, we pose some open problems stronger than the above mentioned conjectures. Moreover, we prove Ban–Linial’s conjecture for cubic cycle permutation graphs. ∗The research that led to the present paper was partially supported by a grant of the group GNSAGA of INdAM and by the Italian Ministry Research Project PRIN 2012 “Geometric Structures, Combinatorics and their Applications”. †Supported by a Postdoctoral Fellowship of the Research Foundation Flanders (FWO). Email addresses: [email protected] (M. Abreu), [email protected] (J. Goedgebeur), [email protected] (D. Labbate), [email protected] (G. Mazzuoccolo) 1 ar X iv :1 70 5. 06 92 8v 1 [ m at h. C O ] 1 9 M ay 2 01 7 As a by–product of studying 2–edge colourings of cubic graphs having linear forests as monochromatic components, we also give a negative answer to a problem posed by Jackson and Wormald about certain decompositions of cubic graphs into linear forests.