1. A question of B. Plotkin about the semigroup of endomorphisms of a free group;Formanek, Edward;Proc. Amer. Math. Soc.,2002
2. B. I. Plotkin, Seven Lectures on the Universal Algebraic Geometry, Preprint, Institute of Mathematics, Hebrew University, 2000.
3. Endomorphisms of finite full transformation semigroups;Schein, Boris M.;Proc. Amer. Math. Soc.,1998
4. \textcyr{B. M. Shaĭn}, \textcyr{Atomnye polugrudy i involyutirovannye polugruppy}, \textcyr{Izvestiya Vyssh. Uchebn. Zaved., Matematika}, 1965, no. 3, 172–184. In Russian: B. M. Schein, Atomic semigroups and involuted semigroups, Izvestiya Vyssh. Uchebn. Zaved. (Matematika), 1965, no. 3, 172–184.
5. J. Schreier, Über Abbildungen einer abstrakten Menge auf ihre Teilmengen, Fundamenta Mathematica 28(1936), 261-264.