Cohomology and homotopy
In higher category theory
A -category is nothing but a coreflective subcategory and a -category is nothing but a reflective subcategory. Since both of these encode reflective localizations, following Rosenberg the “Q” is for quotient and is to indicate that in this context one is interested in notions similar to, but different from, the standard notion of sheaves:
for a -category there is canonically induced a quadruple of adjoint functors between the corresponding presheaf categories
and a presheaf is called an -sheaf if the canonical morphism
is an isomorphism in . More generally, there are generalizations of this condition where presheaves of sets can be replaced with presheaves with values in other categories, notably in abelian categories.
In a central motivating class of examples is a category of sieves on objects in a small category that are regarded as being covering but which do not necessarily satisfy the axioms of a Grothendieck topology and not even of a coverage.
The Yoneda embedding is continuous but not cocontinuous functor. Hence the Grothendieck topologies are used to define smaller image than the category of presheaves such that for that embedding some covering cones will stay covering cones. In one case the cones correspond to the Grothendieck topology but more general families of diagrams may be involved. The important properties of categories of diagrams for doing the sheaf theory can be expressed in terms of certain adjoint pairs of functors. As one application this generalization of sheaf theory can also rephrase categorically properties like formal smoothness and formal etaleness of functors, and as another puts the sheafification in a framework for which another special case is a construction of the Gabriel-type noncommutative localization.
An almost quotient category, or a -category is
a pair of adjoint functors
with the left adjoint;
such that is full and faithful
In other words, is equipped with an equivalence with a coreflective subcategory of .
In motivating classes of examples and are toposes and is a geometric morphism between them. Therefore one generally speaks of as the direct image and of as the inverse image of the Q-category.
The definition 1 is nothing but the definition of a coreflective subcategory. However the term -category is used when the pair is used in a specific meaning useful to constructions in (generalized) sheaf theory (similarly like presheaf of objects in and contravariant functor to are synonyms, but a different word refers to a different context and intuition).
One sometimes write the above data as .
A morphism of Q-categories from to is a triple where , are functors and is a natural isomorphism of functors. The composition is given by
A transformation of morphisms of Q-categories is a pair of natural transformations and such that the diagram
Small Q-categories, morphisms of Q-categories and natural transformations of morphisms form a 2-category of small Q-categories.
A -category is a pair of functors , where is fully faithful and right adjoint to . In other words, is equipped with an equivalence with a reflective subcategory of .
(Co)Presheaves on a -category
The category of presheaves over any -category canonically inherits itself the structure of a Q-category.
Let be a -category. Let be any category.
Then forming copresheaves with values in yields a Q-category of the form
The -unit is induced by the original unit
and the counit is induced by the original counit
The only thing is who is adjoint – now is the left adjoint.
The triangle identities for can be obtained by expanding. For , one has is given by , and for one has . Then has the components given by . Thus for each functor , the composition
is the identity by the triangle identity for , but this is precisely the -component of the transformation
Similarly the -component of
for a functor reads
and this composition equals by another triangle identity for .
It is clear that if is iso then the composition with is also iso. Thus we obtain a -categories.
In other words, since the left adjoint being a full and faithful functor is equivalent to the unit of the adjunction being an isomorphism, it follows from being full and faithful that is full and faithful.
This appears as (Kontsevich-Rosenberg, 2.7).
Any subcategory containing determines a Q-subcategory .
Domain and codomain fibration
For any category , write for its arrow category. This comes equipped with the codomain fibration and the domain cofibration . Both have a joint section by a full and faithful functor that assigns identity morphisms. These form a triple of adjoint functors
Taking this apart, we have that
is a -category and
is a -category.
This appears as (Kontsevich-Rosenberg, 2.5).
Several classes of examples of Q-categories of interest arise as sub-Q-categories of those of this form. For instance the Q-categories of infinitesimal thickenings below.
Passing to copresheaf Q-categories as in prop. 1 we have the Q-category
where the extra right adjoint is given by precomposition with the domain evaluation.
Q-category of cones
Let be a category and be the category whose objects are cones over (small) diagrams where are variable small categories; and the morphisms from to are triples of the form where is a morphism in , is a diagram (= functor), and is a morphism of diagrams (= natural transformation) such that
commutes and denotes the horizontal (= Godement) composition of natural transformations.
Then one defines composition of morphisms by the formula
There is a fully faithful functor that to any assigns the trivial cone and to any morphism the corresponding morphism of trivial cones. Its right adjoint is the morphism defined by sending the cone over a diagram its vertex and to a cone morphism , the morphism of vertices . Then . The identity transformation can be thus taken as the unit of the adjunction. The counit of the adjunction is constructed as follows: to a cone assign the morphism where is the constant diagram which is the unique diagram from to the final category . One can check that these data indeed define an adjoint pair of functors. is therefore a Q-category, and it is called the Q-category of cones.
If is a family of small categories, then one considers the full subcategory of cones whose domains are in ; the rest of the construction restricts to obtain a Q-category .
The most classical case is when is the (say skeletal) category of small discrete categories (=just identity morphisms), one obtains then the Q-category . A semicosite (or semicositus pl. semicositi) is a Q-category of the form where is a full subcategory of and the adjoint pair is obtained by the restriction. A semicosite is a precosite (=Grothendieck precotopology) if
(i) whenever .
(ii) and then
(iii) and , then the family of pushouts exists and belongs to .
An example of a cosite is a cosite of closed sets of a topological space.
The -category of sieves.
The -subcategory of the -category of (all) sieves corresponding to the subcategory of sieves corresponding to the Grothendieck topology…
The Q-category factoring a fully faithful factor
Any fully faithful functor among small categories factors canonically into the composition where is the full subcategory of whose objects are all in such that is a representable functor , and is the corestriction of to . This corestriction makes sense: is fully faithful, hence , i.e. for all in . For each , define now as the functor representing , i.e. by (KR NcSpaces A1.1.1). This relation on objects extends to an adjunction with fully faithful.
Let be a category and a map that assigns to every object a collection of cosieves on that object (subfunctors of ).
Write for the category whose objects are cosieves on , and whose morphisms are morphisms in that respect the corresponding cosieves. This yields a Q-category
Call this a quasi-cosite if
for any two cosieves in their intersection is also in ;
for any cosive in any cosieve containing it is also in .
This is (KontsevichRosenberg, 2.2).
For any Q-category , the quasi-cosite associated with is the Q-category defined by…
The following is supposed to be the standard quasi-cosite for non-commutative geometry.
Let be a ring and the category of associative algebras over . Let be the full subcategory of the domain fibration whose objects are the faithfully flat morphisms, i.e. those morphisms in such that the induced
is an exact and full and faithful functor. This forms a Q-category.
Write for the quasi-cosite associated with this Q-category by def. 7. This is the standard quasi-cosite for noncommutative geometry.
This is (KontsevichRosenberg, A.1.9.2).
Generally, infintiesimal thickenings are characterized by coreflective embeddings:
A characteristic property of an infinitesimally thickened point is that for any object without infinitesimal thickening, there are in general nontrivial morphisms , but there is only a unique morphism , reflecting the fact that has only a single global point. Thus if by we denote the inclusion of objects without infinitesimal thickening into the collection of possibly infinitesimally thickened objects, and by the projection that contracts away the infinitesimal extension, we have indeed an adjunction
This general concept is described at infinitesimal neighbourhood site. See also the discussion below at Relation to cohesive toposes.
The following is one realization of this general concept.
Let be a field and be the category of commutative associative algebras over .
Let be the full subcategory of the codomain fibration Q-category from prop. 5 on those morphisms of commutative algebras which are epimorphisms and whose kernel is nilpotent. Then
is a Q-category.
The analogous statement is true with the category of all associative algebras, not necessarily commutative.
This appears as (Kontsevich-Rosenberg, 2.6).
We discuss the notion of a -sheaves on a Q-category .
Let be a Q-category. An object is called an -sheaf if for all the canonical morphism
is an isomorphism (in Set, hence a bijection).
This morphism is given by
where is the -component of the -counit (which is an isomorphism because is a full and faithful functor).
This appears as (Kontsevich-Rosenberg, 3.1.1).
This appears as (Kontsevich-Rosenberg, 3.1.3).
For more details on the canonical morphism appearing here see the section Adjoint quadruples at cohesive topos.
Using the -adjunction-isomorphism we have that the canonical morphism from def. 9 is isomorphic to
for all , where the second map sends every morphism to its adjunct. Using the definition of the first morphism from def. 9 and the expression of adjuncts (as discussed there) by composition with (co)-units, we find that the composite map here sends any morphism
to the composite
Using that adjunction units are natural transformations, we can complete this to a commuting diagram
This shows that acts on any by postcomposition with the canonical morphism . By the Yoneda lemma it follows that is an isomorphism for all precisely if is an isomorphism.
Let be a Q-category with an extra right adjoint as in prop. 3.
an object is -monopresheaf if is a monomorphism in .
an object is -epipresheaf if is an epimorphism in .
This appears as Kontsevich-Rosenberg, 3.1.2, 3.1.4.
Let be a Q-category where is a small category, and let be a category with all small limits. Then the Q-category of presheaves from prop 1 has an extra right adjoint
given by the right Kan extension along , which exists by the assumption that has all small limits.
Therefore by prop. 3 a presheaf is a -sheaf, def. 9, precisely if the canonical morphism
is an isomorphism.
This appears as (Kontsevich-Rosenberg, 3.5).
Formal smoothness and -sheaves
be the Q-category of infinitesimal thickenings from prop. 2. Write
be the corresponding Q-category of copresheaves from prop. 1. Notice that is the presheaf topos that contains schemes over . Recall the notion of formally smooth scheme, formally étale morphism and formally unramified morphism of schemes.
An object is
formally étale precisely if it is an -sheaf in the sense of def. 9, hence precisely if the canonical morphism
from prop. 3 is an isomorphism;
formally unramified precisely if it is a monopresheaf, def. 1, hence precisly if is a monomorphism;
formally smooth precisely if it is a strict epipresheaf, def. 1, hence precisely if is a strict epimorphism.
This appears as (Kontsevich-Rosenberg, 4.1).
This category theoretic reformulation of these three properties therefore admits straightforward generalization of these notions to other contexts. See the section Infinitesimal paths at cohesive (∞,1)-topos.
For instance we have the following direct generalization is of interest in noncommutative geometry.
Let be the full category of associative algebras over , not necessarily commutative. Write for the Q-category of infinitesimal thickenings as in def. 2. Notice that is the presheaf topos that contains noncommutative schemes.
We then say an object is
This appears as (Kontsevich-Rosenberg, section 4.2).
Let and write for the corresponding representable functor. We have that
is an epipresheaf (formally smooth) precisely if is Quillen-Cuntz quasi-free: the -module , being the kernel of the multiplication morphism
is a projective in Mod;
is an -monopresheaf (formally unramified) precisely if .
This appears as (Kontsevich-Rosenberg, prop. 4.3).
Relation to other concepts
Relation to cohesive toposes
If a Q-category has the extra right adjoint of prop. 3 and in addition an extra left adjoint to a total of a quadruple of adjoint functors
then essential axioms characterizing a cohesive topos are satisfied, in particular if for instance and are presheaf toposes as in 1 (this is considered around Kontsevich-Rosenberg, 3.5.1).
Notably in this case the canonical natural transformation
from prop. 3 is the one appears in the axioms of a cohesive topos: if this transformation is a monomorphism in a cohesive topos – hence if in the language of Q-categories all objects are monopresheaves – one says that discrete objects are concrete in the cohesive topos.
Moreover, due to the extra left adjoint there is a canonical dual morphism
In (Lawvere) is the suggestion that it is interesting to consider the full subcategory of on which is an isomorphism. This is dual to the statement of the above section on A-Sheaves which asserts that it is interesting to consider the full subcategory of on which is an isomorphism.
More concretely, the axioms of Infinitesimal cohesion are an abstraction of the situation of prop. 2. In every cohesive -topos equipped with an infinitesimal neighbourhood there is an analog of the characterization of formal smoothness from prop. 4. See the section Infinitesimal paths and de Rham spaces.
Sheafification versus the Gabriel localization
The term -category originally was short for almost quotient category , presumably because of its role in the theory of Gabriel localization. The sheaf formalism in terms of -categories has been introduced in the mimeographed notes
- Alexander Rosenberg, Q-categories, sheaves and localization, (in Russian) Seminar on supermanifolds 25, Leites ed. Stockholms Universitet 1988 (there is a recent partial English translation)
The formalism is used (and briefly surveyed) in
and also used in the general definition of “noncommutative” stacks in
The epipresheaf condition for the Q-category of nilpotent (infinitesimal) thickenings is in the Kontsevich-Rosenberg paper interpreted as formal smoothness what is further studied in
- T. Brzeziński, Notes on formal smoothness, in: Modules and Comodules (series Trends in Mathematics). T Brzeziński, JL Gomez Pardo, I Shestakov, PF Smith (eds), Birkhäuser, Basel, 2008, pp. 113-124 (doi, arXiv:0710.5527)
The condition that is an isomorpophism , dual to the condition for -sheaves considered above, has been considered in
- Bill Lawvere, Axiomatic cohesion Theory and Applications of Categories, Vol. 19, No. 3, 2007, pp. 41–49. (pdf)