COMBINATORIAL PROPERTIES AND HOMOMORPHISMS OF SEMIGROUPS
Andrei V. Kelarev · International Journal of Algebra and Computation · 1994
A complete description of ultrarepetitive semigroups is given. As an application of this result all semigroups S are described such that the class of permutational semigroups is S-closed, i.e., closed under taking preimages of certain homomorphisms onto S.