1. B. I. Plotkin, L. Ja. Greenglaz, and A. A. Gvaramija, Algebraic Structures in Automata and Databases Theory (World Scientific, Singapore, 1992).
2. V. A. Molchanov, ‘‘Semigroups of mappings on graphs,’’ Semigroup Forum 27, 155–199 (1983).
3. S. A. Akimova, ‘‘Concrete characterization of input symbol semigroup of universal ordered automata,’’ Nauka Obshch. 1 (1), 35–38 (2011).
4. E. V. Khvorostukhina, ‘‘On homomorphisms of endomorphism semigroups of hypergraphs,’’ Izv. Saratov Univ., Mat. Mekh. Inform. 3 (9), 70–75 (2009).
5. S. M. Ulam, A Collection of Mathematical Problems (Interscience, New York, 1960).