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(Tk,3) = ⌈(3/2)(k+1)⌉, where Tk,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.
Famous quotes containing the word harmonious:
“Even in harmonious families there is this double life: the group life, which is the one we can observe in our neighbour’s household, and, underneath, another—secret and passionate and intense—which is the real life that stamps the faces and gives character to the voices of our friends. Always in his mind each member of these social units is escaping, running away, trying to break the net which circumstances and his own affections have woven about him.”
—Willa Cather (1873–1947)