**Problem**The valency-variety of a graph is the number of different degrees in . Is the chromatic number of any graph with at least two vertices greater than

According to Jensen and Toft [JT, p. 90], the problem is due to Melnikov and was mentioned by Vizing [V] and Zykov [Z]. According to Zykov [Z], Melnikov showed that the suggested lower bound would be best possible.

A best possible upper bound on the chromatic number in terms of and is as proved by Nettleton [N] and Dirac [D].

## Bibliography

[D] G. A. Dirac. Valency-variey and chromatic number of abstract graphs. Wiss. Z. Martin-Luther-Univ. Halle-Wittenberg Math.-Natur. Reihe 13, 59--64, 1964.

[JT] Tommy R. Jensen, Bjarne Toft: Graph Coloring Problems, Wiley-Interscience Series in Discrete Mathematics and Optimization. John Wiley & Sons Inc., New York, 1995.

[N] R. E. Nettleton. Some generalized theorems on connectivity. Canad. J. Math. 12, 546--554, 1960.

*[V] V. G. Vizing. Some unsolved problems in graph theory (in Russian). Uspekhi Mat. Nauk. 23, 117--134, 1968. English translation in Russian Math. Surveys 23, 125--141.

*[Z] A. A. Zykov. Problem 11. In: H. Sachs, H.-J. Voss, and H. Walther, editors, Beiträge zur Graphentheorie vorgetragen auf dem Internationalen Kolloquium in Manebach DDR vom 9.-12. Mai 1967, page 228. B. G. Teubner, 1968.

* indicates original appearance(s) of problem.