1. Some unsolved problems;Erdős;Magyar Tud. Akad. Mat. Kut. Int. Kozl.,1961
2. Crucial words and the complexity of some extremal problems for sets of prohibited words;Evdokimov;J. Combin. Theory Ser. A,2004
3. A. Glen, B.V. Halldórsson, S. Kitaev, Crucial words for abelian powers, in: Proceedings of the 13th International Conference on Developments in Language Theory, DLT 2009, Stuttgart, Germany, June 30–July 3, 2009