# Graph colouring

An assignment of colours to the vertices and/or edges of a graph which displays certain properties. A regular vertex (edge) colouring is a colouring of the vertices (edges) of a graph in which any two adjacent vertices (edges) have different colours. A regular vertex colouring is often simply called a graph colouring. A graph is said to be -colourable if there exists a regular vertex colouring of the graph by colours. The smallest number of colours which suffices for a regular vertex colouring of a graph is called the chromatic number of . If , is said to be -chromatic. A graph is -chromatic if and only if it contains no simple cycles of odd length. If the maximum degree of the vertices of is , is always -colourable except in two cases: 1) and has a connected component which is a cycle of odd length; or 2) and the complete graph with vertices is a connected component of .

The following inequality is valid for the union of two graphs and :

equality being attainable. Moreover, if is such that , then there exist subgraphs and in such that , , . If is a graph with vertices and is the complementary graph to , then

all the bounds being attainable. The chromatic number of a two-dimensional surface is the largest of the chromatic numbers of the graphs which can be regularly imbedded in (cf. Graph imbedding). The equality

is valid for an orientable surface of genus . If , this equation assumes the form , which is the statement of the four-colour problem. Let be the number of different regular colourings of a graph with numbered vertices with or fewer colours; then, for any graph , the function is a polynomial in the variable , called the chromatic polynomial of . Thus, the chromatic polynomial of any tree with vertices has the form . The edge chromatic number (chromatic class) of is the smallest number of colours sufficient for a regular colouring of the edges of . If the maximum degree of the vertices of is (multiple edges are permitted), then

If, in addition, the multiplicity of each edge is at most , then . In particular, for graphs without loops or multiple edges .

Problems involving graph colouring arise in design of communications, in radio-electronics, in planning of experiments, and in other fields.

#### References

[1] | F. Harary, "Graph theory" , Addison-Wesley (1969) pp. Chapt. 9 |

[2] | O. Ore, "Theory of graphs" , Amer. Math. Soc. (1962) |

[3] | C.E. Shannon, "A theorem on colouring the lines of a network" J. Math. Phys. , 28 (1949) pp. 148–151 |

#### Comments

Besides vertex colourings and edge colourings, the colourings of the domains (faces) of a planar map have also been studied. The four-colour problem mentioned in the article above (which is now the four-colour theorem) originally belonged to this category (see Graph, planar). Recent surveys on edge colourings are [a1] and [a2].

#### References

[a1] | S. Fiorini, R.J. Wilson, "Edge-colourings of graphs" , Pitman (1977) |

[a2] | S. Fiorini, R.J. Wilson, "Edge-colourings of graphs" L.W. Beineke (ed.) R.J. Wilson (ed.) , Selected Topics in Graph Theory , Acad. Press (1978) pp. Chapt. 5 |

[a3] | R.J. Wilson, "Introduction to graph theory" , Longman (1985) |

[a4] | R.C. Read, "An introduction to chromatic polynomials" J. Comb. Theory , 4 (1968) pp. 52–71 |

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Graph colouring.

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