Presentations and word problem for strong semilattices of semigroups

Abstract

Let \(I\) be a semilattice, and \(S_i\) \((i\in I)\) be a family of disjoint semigroups. Then we prove that the strong semilattice \(S=\mathcal{S} [I,S_i,\phi _{j,i}]\) of semigroups \(S_i\) with homomorphisms \(\phi _{j,i}:S_j\rightarrow S_i\) (\(j\geq i\)) is finitely presented if and only if \(I\) is finite and each \(S_i\) \((i\in I)\) is finitely presented. Moreover, for a finite semilattice \(I\), \(S\) has a soluble word problem if and only if each \(S_i\) \((i\in I)\) has a soluble word problem. Finally, we give an example of non-automatic semigroup which has a soluble word problem.