Automorphisms of the endomorphism semigroup of a free monoid or a free semigroup

Abstract

We determine all isomorphisms between the endomorphism semigroups of free monoids or free semigroups and prove that automorphisms of the endomorphism semigroup of a free monoid or a free semigroup are inner or “mirror inner". In particular, we answer a question of B. I. Plotkin.