Presentations and word problem for strong semilattices of semigroups

Abstract

Let I be a semilattice, and Si (i ∈ I) be a family of disjoint semigroups. Then we prove that the strong semilattice S = S[I, Si , φj,i] of semigroups Si with homomorphisms φj,i : Sj → Si (j ≥ i) is finitely presented if and only if I is finite and each Si (i ∈ I) is finitely presented. Moreover, for a finite semilattice I, S has a soluble word problem if and only if each Si (i ∈ I) has a soluble word problem. Finally, we give an example of nonautomatic semigroup which has a soluble word problem.