**Problem**Let be a perfect graph on vertices. Is it true that either or contains a complete bipartite subgraph with bipartition so that ?

Every perfect graph on vertices either has a clique or an independent set of size , so weakening the bound on , to gives a true statement. Jacob Fox [F] has proved that every comparability graph on vertices has a complete bipartite subgraph of size , and (up to the constant) this is best possible.

## Bibliography

[F] J. Fox, A Bipartite Analogue of Dilworth’s Theorem, Order 23 (2006), 197-209.

* indicates original appearance(s) of problem.