Some results on embeddings of algebras, after de Bruijn and McKenzie

Abstract

In 1957, N.G. de Bruijn showed that the symmetric group Sym(Ω) on an infinite set Ω contains a free subgroup on 2 card(Ω) generators, and proved a more general statement, a sample consequence of which is that for any group A of cardinality ⩽ card(Ω), the group Sym(Ω) contains a coproduct of 2 card(Ω) copies of A, not only in the variety of all groups, but in any variety of groups to which A belongs. His key lemma is here generalized to an arbitrary variety of algebras V, and formulated as a statement about functors Set ℩ V. From this one easily obtains analogs of the results stated above with “group” and Sym(Ω) replaced by “monoid” and the monoid Self(Ω) of endomaps of Ω, by “associative K-algebra” and the K-algebra End K ( V) of endomorphisms of a K-vector-space V with basis Ω, and by “lattice” and the lattice Equiv(Ω) of equivalence relations on Ω. It is also shown, extending another result from de Bruijn's 1957 paper, that each of Sym(Ω), Self(Ω) and End K ( V) contains a coproduct of 2 card(Ω) copies of itself. That paper also gave an example of a group of cardinality 2 card(Ω) that was not embeddable in Sym(Ω), and R. McKenzie subsequently established a large class of such examples. Those results are shown here to be instances of a general property of the lattice of solution sets in Sym(Ω) of sets of equations with constants in Sym(Ω). Again, similar results - this time of varying strengths - are obtained for Self(Ω), End K ( V), and Equiv(Ω), and also for the monoid Rel(Ω) of binary relations on Ω. Many open questions and areas for further investigation are noted.