Tatjana Petkovic Miroslav Ciric Stojan Bogdanovic Unary algebras, semigroups and congruences on free semigroups. 87-105 2004 324 Theor. Comput. Sci. 1 http://dx.doi.org/10.1016/j.tcs.2004.03.054 db/journals/tcs/tcs324.html#PetkovicCB04