For a universal algebra 𝓐, let End(𝓐) and Aut(𝓐) denote, respectively, the endomorphism monoid and the automorphism group of 𝓐. Let S be a semigroup and let T be a characteristic subsemigroup of S. We say that ϕ ∈ Aut(S) is a lift for ψ ∈ Aut(T) if ϕ|T = ψ. For ψ ∈ Aut(T) we denote by L(ψ) the set of lifts of ψ, that is, $L(ψ) = ϕ ∈ Aut(S) | ϕ|_{T} = ψ}$. Let 𝓐 be an independence algebra of infinite rank and let S be a monoid of monomorphisms such that G = Aut(𝓐) ≤ S ≤ End(𝓐). It is obvious that G is characteristic in S. Fitzpatrick and Symons proved that if 𝓐 is a set (that is, an algebra without operations), then |L(ϕ)| = 1. The author proved in a previous paper that the analogue of this result does not hold for all monoids of monomorphisms of an independence algebra. The aim of this paper is to prove that the analogue of the result above holds for semigroups S = ⟨Aut(𝓐) ∪ E ∪ R⟩ ≤ End(𝓐), where E is any set of idempotents and R is the empty set or a set containing a special monomorphism α and a special epimorphism α*.
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For a universal algebra 𝓐, let End(𝓐) and Aut(𝓐) denote, respectively, the endomorphism monoid and the automorphism group of 𝓐. Let S be a semigroup and let T be a characteristic subsemigroup of S. We say that ϕ ∈ Aut(S) is a lift for ψ ∈ Aut(T) if ϕ|T = ψ. For ψ ∈ Aut(T) we denote by L(ψ) the set of lifts of ψ, that is, L(ψ) = {ϕ ∈ Aut(S) | ϕ|T = ψ}. Let 𝓐 be an independence algebra of infinite rank and let S be a monoid of monomorphisms such that G = Aut(𝓐) ≤ S ≤ End(𝓐). In [2] it is proved that if 𝓐 is a set (that is, an algebra without operations), then |L(ϕ)| = 1. The analogous result for vector spaces does not hold. Thus the natural question is: Characterize the independence algebras in which |L(ϕ)| = 1. The aim of this note is to answer this question.
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Denote by PSelf Ω (resp., Self Ω) the partial (resp., full) transformation monoid over a set Ω, and by Sub V (resp., End V) the collection of all subspaces (resp., endomorphisms) of a vector space V. We prove various results that imply the following: (1) If card Ω ≥ 2, then Self Ω has a semigroup embedding into the dual of Self Γ iff $card Γ ≥ 2^{card Ω}$. In particular, if Ω has at least two elements, then there exists no semigroup embedding from Self Ω into the dual of PSelf Ω. (2) If V is infinite-dimensional, then there is no embedding from (Sub V,+) into (Sub V,∩) and no embedding from (End V,∘) into its dual semigroup. (3) Let F be an algebra freely generated by an infinite subset Ω. If F has fewer than $2^{card Ω}$ operations, then End F has no semigroup embedding into its dual. The bound $2^{card Ω}$ is optimal. (4) Let F be a free left module over a left ℵ₁-noetherian ring (i.e., a ring without strictly increasing chains, of length ℵ₁, of left ideals). Then End F has no semigroup embedding into its dual. (1) and (2) above solve questions proposed by G. M. Bergman and B. M. Schein. We also formalize our results in the setting of algebras endowed with a notion of independence (in particular, independence algebras).
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