Decompositions of Automata and Transition Semigroups

The purpose of this paper is to describe structural properties of automata whose transition semigroups have a zero, left zero, right zero or bi-zero, or are nilpotent extensions of rectangular bands, left zero bands or right zero bands, or are nilpotent. To describe the structure of these automata we use various well-known decomposition methods of automata theory – direct sum decompositions, subdirect and parallel decompositions, and extensions of automata. Automata that appear as the components in these decompositions belong to some well-known classes of automata, such as directable, definite, reverse definite, generalized definite and nilpotent automata. But, we also introduce some new classes of automata: generalized directable, trapped, onetrapped, locally directable, locally one-trapped, locally nilpotent and locally definite automata. We explain relationships between the classes of all these automata.