# General Theory of Left-Exact Localization?

A left-exact localization of a category is a reflective subcategory such that the reflector preserves finite limits. There are several prominent examples of such localizations, such as sheafification, and localization of module categories. Is there a general theory of such localizations?

I don't have any particular type of result in mind, but given the prominence of the two examples I mentioned, it seems like the topic of left-exact localizations must have been studied for its own sake.

• Special cases of reflective localisation in locally presentable categories are studied in § 1.C of [Adámek and Rosický, Locally presentable and accessible categories] and, of course, the theory of left exact localisations of presheaf toposes is just the theory of Grothendieck topologies. – Zhen Lin Apr 23 '13 at 7:47
• Basics of the general theory are reviewed in the first thirteen pages of arXiv:0806.1324 Localization theory for triangulated categories. – dhagbert Apr 23 '13 at 10:00
• I don't see where Krause addresses left-exactness. – arsmath Apr 23 '13 at 10:31
• The first volume of Borceux's "Handbook of categorical algebra" has some discussion of left exact localizations. – Ricardo Andrade Apr 23 '13 at 11:07
• @DavidWhite Questions are off-topic now if they have answers? A surprising development for MO. – arsmath Aug 20 '19 at 21:10

Just to tie this one up, the cited result in Borceux (Prop 5.6.1) says the following:

Proposition: Let $$\mathcal C$$ be a finitely-complete category and let $$L \mathcal C$$ be a reflective subcategory. Let $$\mathcal W$$ be the class of morphisms inverted by the reflector $$r: \mathcal C \to L\mathcal C$$. Then $$r$$ is left exact if and only if $$\mathcal W$$ is stable under base change.

This continues to hold in the $$\infty$$-categorical context. Here is a proof adapted from Borceux which works $$\infty$$-categorically.

Proof: The "only if" direction is clear; we prove "if". First, since $$i$$ preserves terminal objects we have that the terminal object of $$L\mathcal C$$ is the terminal object of $$\mathcal C$$ and in particular is $$\mathcal W$$-local, so $$L$$ preserves terminal objects.

So it will suffice to show that $$L$$ preserves pullbacks. To this end, it is sufficient to show that if we have a natural transformation from the pullback square on the left to the one on the right below, and if the components $$B \xrightarrow \sim B'$$, $$C \xrightarrow \sim C'$$, and $$D \xrightarrow \sim D'$$ are all in $$\mathcal W$$, then so is the map $$A \to A'$$.

$$\require{AMScd} \begin{CD} A @>>> B\\ @VVV @VVV\\ C @>>> D \end{CD} \qquad \Rightarrow \qquad\begin{CD} A' @>>> B'\\ @VVV @VVV\\ C' @>>> D' \end{CD}$$

By pullback-stability and 2/3, the map $$B \times_{D'} C \xrightarrow \sim A'$$ is in $$\mathcal W$$. So by 2/3 it will suffice to show that the map $$A \to B \times_{D'} C$$ is in $$\mathcal W$$. By pullback-stability and 2/3, the map $$D \xrightarrow \sim D \times_{D'} D$$ is in $$\mathcal W$$, so it will suffice by pullback-stability to show that the following two squares are pullbacks:

$$\begin{CD} A @>>> B @>>> D \\ @VVV @VVV @VV^\sim V \\ B \times_{D'} C @>>> B \times_{D'} D @>>> D \times_{D'} D \end{CD}$$

This can be seen using the following two diagrams:

$$\begin{CD} A @>>> B \\ @VVV @VVV \\ B \times_{D'} C @>>> B \times_{D'} D @>>> B \\ @VVV @VVV @VVV \\ C @>>> D @>>> D' \end{CD} \quad \begin{CD} B @>>> D \\ @VVV @VVV \\ B \times_{D'} D @>>> D \times_{D'} D @>>> D \\ @VVV @VVV @VVV \\ B @>>> D @>>> D' \end{CD}$$

In each case we argue that the bottom-right square and the composite of the lower two squares is a pullback, so the bottom-left square is a pullback. Since the composite of the left two squares is also a pullback, it results that the top-left square is a pullback.

This is my bibliography on the subject. Indeed Bourceux was quite relevant in this topic.

• Borceux, Sheaves of algebras for a commutative theory, Ann. Soc. Sci. Bruxelles Sér. I 95 (1981), no. 1, 3–19
• Borceux and Kelly, On locales of localizations, J. Pure Appl. Algebra, Volume 46, Issue 1, 1987, Pages 1-34.
• Borceux and Veit, On the Left Exactness of Orthogonal Reflections J. Pure Appl. Algebra, 49 (1987), pp. 33-42.
• Borceux, Subobject Classifier for Algebraic Structures. Subobject classifier for algebraic structures J. Algebra, 112 (1988), pp. 306-314.
• Veit, Sheaves, localizations, and unstable extensions: Some counterexamples. J. Pure Appl. Algebra, Volume 140, Issue 2, July 1991, Pages 370-391.
• Borceux and Quinteiro. A theory of enriched sheaves. Cahiers de Topologie et Géométrie Différentielle Catégoriques 37.2 (1996): 145-162.
• Garner and Lack, Lex Colimits. J. Pure Appl. Algebra. Volume 216, Issue 6, June 2012, Pages 1372-1396.
• I think <<Cassidy, C.; Hébert, M.; Kelly, G. M. Reflective subcategories, localizations and factorization systems. J. Austral. Math. Soc. Ser. A 38 (1985), no. 3, 287--329.>> also belongs on this list. – Alexander Campbell Jan 8 at 0:28