1. J. Brzozowski, Open Problems about regular languages, in normal Languages Perspectives and Open Problems, R. Book editor, Academic press, New York, 1980.
2. Y. Cesari, M.Vincent, Une caracterisation des mots périodiques, C.R.Acad. Sci. Paris, 286, A, 1175–1177.
3. J.P. Duval, Périodes et répétitions des mots du monoide libre, 1979 Theoret. Comp. Sci., 9, 17–26.
4. S. Eilenberg, Automata Languages and Machines, Vol.b, 1976, Academic Press, New York.
5. G. Lallement, Semigroups and Combinatorial applications, 1979, Wiley Interscience.