The orthogonal subcategory problem for a class of morphisms in a category asks whether the full subcategory of objects orthogonal to is a reflective subcategory.
Here, an object is said to be orthogonal to a map if every morphism factors uniquely through . For example, in a category of presheaves, is orthogonal to the canonical map if it preserves the limit in .
This problem is related to the problem of localization. Suppose is indeed a reflective subcategory; let be the reflector (the left adjoint to the inclusion ).
If belongs to , then is an isomorphism in .
By definition of orthogonality, for every object in , induces an isomorphism of hom-sets
Since , this means that for all in the map
is an isomorphism, so that is a natural isomorphism between representables. By the Yoneda lemma, this means is an isomorphism.
This lemma can be sharpened. First, given a category , there is a Galois connection between classes of morphisms and classes of objects , induced by the predicate that is a bijection for all and all . The induced monad on the collection of morphism classes (partially ordered by inclusion) may be called “saturation”, denoted . An easy argument due to Gabriel-Zisman is that if is finitely cocomplete, then satisfies the axioms for a calculus of fractions, so that the localization can be constructed. An easy argument then establishes the following sharpened lemma:
If is reflective, then the reflection exhibits a canonical equivalence
If is reflective, then the reflection exhibits a canonical equivalence
To be connected with things like small object argument, Bousfield localization, and others…
If is a set of morphisms in a locally presentable category then the orthogonality subcategory is reflective in .
Peter Freyd, Max Kelly, Categories of continuous functors I, Jour. Pure Appl. Alg. 2 (1972), 169-191. (web)
Jiří Adámek, Jiří Rosický, Locally presentable and accessible categories, Cambridge University Press, (1994). But see the errata.
Last revised on August 19, 2019 at 20:16:35. See the history of this page for a list of all contributions to it.