A note on unification type zero
An equational theory E is of unification type zero if there exist terms s, t for which there is no minimal complete set of E-unifiers of s, t. The existence of such a theory E was conjectured by Plotkin in 1972. The first example of a theory of this type was constructed by Fages and Huet in 1983. The present paper shows that the theory of idempotent semigroups is of unification type zero. This yields an example of a theory of type zero which is more natural than the example given by Fages and Huet.
scientific article; zbMATH DE number 4037183