1. J.A.Anderson, The intersection of retracts of A ; Theoretical Computer Science, 2000, 237, pp. 312-326.
2. J.A.Anderson,W.Forys, T.Head, Retracts and semiretracts of free monoids, AMS Meeting, San Francisco, 1991.
3. S.Eilenberg, Automata, Languages, and Machines, Academic Press, 1974.
4. W.Forys, K.Krawczyk, An Algorithmic Approach to the Problem of a Semiretract Base, Theoretical Computer Science, 2006, 369, pp. 314-322.
5. G.Paun A.Salomaa, Semi-commutativity Sets a Cryptographically Grounded Topic, Bull. Math. Soc. Sci. Math. Roumaine, 1991, 35, pp. 255-270.