A connected simple graph is called double-critical, if removing any pair of adjacent vertexes lowers the chromatic number by two.

**Conjecture**is the only -chromatic double-critical graph

This conjecture is a special case of a more general problem by Erdos and Lovasz proposed in 1966. It has been independently proven for the case where by Mozhan [3] and Stiebitz [4].

## Bibliography

*[1] P. Erdos, Problem 2, in: Theory of Graphs (Proc. Colloq., Tihany, 1966), Academic Press, New York, 1968, p. 361.

[2] F. Chung, R. Graham, Erdos on graphs: His legacy of unsolved problems, A K Peters, Wellesley, Massachusetts, 1998.

[3] N. N. Mozhan, On double critical graphs with the chromatic number five, Metody Diskretb. Anal. 46 (1987) 50-59.

[4] M. Stiebitz, is the only double-critical -chromatic graph, Discrete Math. 64 (1987) 91-93.

* indicates original appearance(s) of problem.