HNN extensions of semigroups
The paper gives a new proof of a result of Howie which says that if \(A\) and \(B\) are isomorphic unitary subsemigroups of a semigroup \(S\) then \(S^ 1\) is embeddable in an HNN type extension in which \(A\) and \(B\) are conjugate by virtue of an introduced stable letter \(t\). The proof uses a result of Baer which characterizes those partial groupoids that are semigroup embeddable.
scientific article; zbMATH DE number 611117