[1] D. Jakubíková-Studenovská: Systems of unary algebras with common endomorphisms I, II. Czech. Math. J. 29 (104) 1979, 406-420, 421-429. MR 0536068

[2] D. Jakubíková-Studenovská: Endomorphisms and connected components of partial monounary algebras. Czech. Math. J. 35 (110) 1985, 467-490. MR 0803041

[3] O. Kopeček: Homomorphisms of partial unary algebras. Czech. Math. J. 26 (101) 1976, 108-127. MR 0392759

[4] O. Kopeček M. Novotný: On some invariants of unary algebras. Czech. Math. J. 24 (99) 1974, 219-246. MR 0347703

[5] E. Nelson: Homomorphisms of monounary algebras. Pacif. J. Math., 99, 1982, No 2, 427 to 429. DOI 10.2140/pjm.1982.99.427 | MR 0658072

[6] M. Novotný: Sur un problème de la théorie des applications. Publ. Fac. Sci. Univ. Masaryk, No 344, 1953, 53-64. MR 0061149

[7] M. Novotný: Über Abbildungen von Mengen. Pacif. J. Math. 13, 1963, 1359-1369. DOI 10.2140/pjm.1963.13.1359 | MR 0157143