Harmonious coloring

Harmonious coloring


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 χH("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 χH("G") ≤ |V(G)|. There trivially exist graphs "G" with χH("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 χH("G"):
# χH(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 Frank, Harary and Plantholt (1982).Still very little is known about it.

See also: Complete coloring

External links

* [http://www.maths.dundee.ac.uk/~kedwards/biblio.html] "A Bibliography of Harmonious Colourings and Achromatic Number" by Keith Edwards

References

* Frank, O.; Harary, F.; Plantholt, M. (1982). The line-distinguishing chromatic number of a graph. "Ars Combin." 14, 241–252.
* Jensen, Tommy R.; Toft, Bjarne (1995). "Graph coloring problems". New York: Wiley-Interscience. ISBN 0-471-02865-7.
* Mitchem, J. (1989). On the harmonious chromatic number of a graph. "Discrete Math." 74, 151–157.


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