On the small graphs with chromatic number 5 without 4-cliques

Tort in [4] drew attention to the problem of the minimum number of vertices of the 5-chromatic graphs, not containing 4-cliques. Hanson et al. in [1] solve this problem by proving that the minimum number of vertices equals 11. But perhaps they were not aware that in paper of Nenov [3] given in the references under N 3 by them, the problem has already been solved. The aim of this note is to draw attention to the fact that: Firstly, the graph shown in Fig. 1, of [1] found by Youngs is in fact published in [2] (see p. 165) by Nenov [3].