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Preservation of bi-endomorphic composition by categorical reflection. (English) Zbl 0674.08006
Given a concrete category \({\mathcal C}\) over Set (with forgetful functor U), a bi-endomorphic composition on a \({\mathcal C}\)-object A is a binary operation on the underlying set U(A) such that both exponential transposes \(U(A) \rightrightarrows [U(A),U(A)]\) factor through the hom- set \({\mathcal C}(A,A)\). If \(\bar {\mathcal C}\) is a (full) reflective subcategory of \({\mathcal C}\), one may ask whether the two compositions \(U(A) \rightrightarrows {\mathcal C}(A,A)\to{\mathcal C}(\bar A,\bar A)\) factor through the underlying map of the universal morphism \(A\to \bar A\), and if they do, whether the resulting functions \(U(\bar A) \rightrightarrows {\mathcal C}(\bar A,\bar A)\to [U(\bar A),U(\bar A)]\) are in fact the exponential transposes of some bi-endomorphic composition on \(\bar A.\) The author uses this approach to compare reflections in categories \({\mathcal C}\) and \({\mathcal C}'\) of partial algebras and domain-preserving homomorphisms, where the \({\mathcal C}'\)-objects are \({\mathcal C}\)-objects with an additional total bi-endomorphic composition. In good cases, the reflection in \({\mathcal C}'\) reduces to the reflection in \({\mathcal C}\), thus eliminating the need for a separate construction. As an example, the relatively free \(\Sigma\)- ring can simply be obtained by forming the corresponding relatively free \(\Sigma\)-group.
Reviewer: J.Koslowski

MSC:
08C05 Categories of algebras
18A40 Adjoint functors (universal constructions, reflective subcategories, Kan extensions, etc.)
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