1. Černý, J.: Poznámka k homogénnym eksperimentom s konečnými automatami. Matematicko-fyzikálny Časopis Slovensk. Akad. Vied 14(3) 208–216 (1964) (in Slovak)
2. Eppstein, D.: Reset sequences for monotonic automata. SIAM J. Comput. 19, 500–510 (1990)
3. Garey, M.R., Johnson, D.S.: Computers and Intractability: A Guide to the Theory of NP-completeness. Freeman, San Francisco (1979)
4. Gawrychowski, P.: Complexity of the approximation of the shortest synchronizing word. In: Workshop “Around the Černý Conjecture”. Univ. Wrocław (2008) (unpublished)
5. Goralčik, P., Koubek, V.: Rank problems for composite transformations. Int. J. Algebra and Computation 5, 309–316 (1995)