Introduction Few things influence us as much as colour and few things engage and interest people as much as colour. Colour plays an essential role in the environment in which we live, whether it is at our place of work, in the public environment or in our home. To choose the right colour is necessary to attain the desired effect in colouring and design, both in the colouring of our environment ...

A harmonious colouring of a simple graph G is a proper vertex colouring such that each pair of colours appears together on at most one edge. The harmonious chromatic number h(G) is the least number of colours in such a colouring. It was shown by Hopcroft and Krishnamoorthy (1983) that the problem of determining the harmonious chromatic number of a graph is NP-hard. We show here that the problem...

An edge colouring of a multigraph can be thought of as a partition of the edges into matchings (a matching meets each vertex at most once). Analogously, an edge cover colouring is a partition of the edges into edge covers (an edge cover meets each vertex at least once). We aim to determine a tight lower bound on the maximum number of parts in an edge cover colouring as a function of the minimum...

A vertex colouring assigns to each vertex of a graph a colour such that adjacent vertices have different colours. The algorithmic complexity of the Colouring problem, asking for the smallest number of colours needed to vertex-colour a given graph, is known for a large number of graph classes. Notably it is NP-complete in general, but polynomial time solvable for perfect graphs. A triple of vert...

Behaviour Types The original semantics of Reo is defined in terms of abstract behaviour types [8], which are co-inductively defined relations over timed data streams (a timed stream paired with a value stream). These models account for synchronisation, dataflow, and, implicitly, state changes. They mention neither context dependency nor reconfiguration. This semantics provide little guidance fo...

A harmonious colouring of a simple graph G is a proper vertex colouring such that each pair of colours appears together on at most one edge. The harmonious chromatic number h(G) is the least number of colours in such a colouring. We obtain a new lower bound for the harmonious chromatic number of general graphs, in terms of the independence number of the graph, generalizing results of Moser [2].

We present a polynomial algorithm, implicit in the work of ElZahar and Sauer, which inputs a 3-colouring of a categorical product of two graphs and outputs a 3-colouring of one of the factors. We raise a question about the existence of polynomial algorithms for colouring the vertices of some graphs in terms of intrinsic succint description of the vertices rather than in terms of the (exponentia...

Given a graph G and a positive integer p, χp(G) is the minimum number of colours needed to colour the vertices of G so that for any i ≤ p, any subgraph H of G of tree-depth i gets at least i colours. This paper proves an upper bound for χp(G) in terms of the k-colouring number colk(G) of G for k = 2p−2. Conversely, for each integer k, we also prove an upper bound for colk(G) in terms of χk+2(G)...

A graph G is chordless if no cycle in G has a chord. In the present work we investigate the chromatic index and total chromatic number of chordless graphs. We describe a known decomposition result for chordless graphs and use it to establish that every chordless graph of maximum degree ∆ ≥ 3 has chromatic index ∆ and total chromatic number ∆+1. The proofs are algorithmic in the sense that we ac...