Harmonious coloring

File:Harmonious coloring tree.svg

Harmonious coloring of 7-tree with 3 levels using 12 colors. The harmonius chromatic number of this graph is 12 since the vertices are 57, and the color's pair are ncolor*(ncolor-1)/2 >= 57 iff ncolor>=12. Moreover (3/2)*(7+1)=12(see Mitchem's Formula).

In graph theory, a harmonious coloring is a (proper) vertex coloring in which every pair of colors appears on at most one pair of adjacent vertices. The harmonious chromatic number χ_H(G) of a graph G is the minimum number of colors needed for any harmonious coloring of G.

Every graph has a harmonious coloring, since it suffices to assign every vertex a distinct color; thus χ_H(G) ≤ |V(G)|. There trivially exist graphs G with χ_H(G) > χ(G) (where χ is the chromatic number); one example is the path of length 2, which can be 2-colored but has no harmonious coloring with 2 colors.

Some properties of χ_H(G):

1. χ_H(T_k,3) = ⌈(3/2)(k+1)⌉, where T_k,3 is the complete k-ary tree with 3 levels. (Mitchem 1989)

Harmonious coloring was first proposed by Harary and Plantholt (1982). Still very little is known about it.

See also

• Complete coloring
• Harmonious labeling

External links

• A Bibliography of Harmonious Colourings and Achromatic Number by Keith Edwards

References

• Jensen, Tommy R.; Toft, Bjarne (1995). Graph coloring problems. New York: Wiley-Interscience. ISBN 0-471-02865-7.