Kaluzhnin-Krasner embedding theorem for monoids

Abstract

We study Schreier extensions of monoids and establish a Kaluzhnin--Krasner embedding theorem for Schreier extensions. First, we prove that the category of monoids is not locally algebraically cartesian closed (LACC) and that a monoid is algebraically exponentiable in the category of monoids if and only if it is a Dedekind-finite monoid. Second, we recall that the category of extensions of monoids is $S$-LACC with $S$ the class of Schreier extensions, which defines a wreath product $A \wr B$ for any two monoids. Finally, we prove a Kaluzhnin-Krasner embedding theorem for Schreier extensions that are not necessarily split, i.e. given any Schreier extension $A \hookrightarrow G \twoheadrightarrow B$ of monoids, there is a monomorphism $\phi_G \colon G \hookrightarrow A \wr B$, which is part of a morphism of extensions. The proof adapts the classical group-theoretic argument by replacing conjugation, which requires inverses, with a substitute made available by the Schreier property, namely, the unique factorization of elements in the fibers of the projection $p \colon G \twoheadrightarrow B$.