Ind-completion

In mathematics, the ind-completion or ind-construction is the process of freely adding filtered colimits to a given category C. The objects in this ind-completed category, denoted Ind(C), are known as direct systems, they are functors from a small filtered category I to C.

The dual concept is the pro-completion, Pro(C).

Definitions

Filtered categories

Further information: Filtered category

Direct systems depend on the notion of filtered categories. For example, the category N, whose objects are natural numbers, and with exactly one morphism from n to m whenever $n\le m$, is a filtered category.

Direct systems

A direct system or an ind-object in a category C is defined to be a functor

$F:I\to C$

from a small filtered category I to C. For example, if I is the category N mentioned above, this datum is equivalent to a sequence

$X_0\to X_1\to \cdots$

of objects in C together with morphisms as displayed.

The ind-completion

Ind-objects in C form a category ind-C.

Two ind-objects

$F:I\to C$

and

$G:J\to C$ determine a functor

I^op x J $\to$ Sets,

namely the functor

${\mathrm{Hom}}_C(F(i),G(j)).$

The set of morphisms between F and G in Ind(C) is defined to be the colimit of this functor in the second variable, followed by the limit in the first variable:

${\mathrm{Hom}}_{\mathrm{Ind}\text{-}C}(F,G)=\underset{i}{\lim }{\mathrm{colim}}_j{\mathrm{Hom}}_C(F(i),G(j)).$

More colloquially, this means that a morphism consists of a collection of maps $F(i)\to G(j_i)$ for each i, where $j_i$ is (depending on i) large enough.

Relation between C and Ind(C)

The final category I = {*} consisting of a single object * and only its identity morphism is an example of a filtered category. In particular, any object X in C gives rise to a functor

$\{*\}\to C,*\mapsto X$

and therefore to a functor

$C\to \mathrm{Ind}(C),X\mapsto (*\mapsto X).$

This functor is, as a direct consequence of the definitions, fully faithful. Therefore Ind(C) can be regarded as a larger category than C.

Conversely, there need not in general be a natural functor

$\mathrm{Ind}(C)\to C.$

However, if C possesses all filtered colimits (also known as direct limits), then sending an ind-object $F:I\to C$ (for some filtered category I) to its colimit

${\mathrm{colim}}_{I}F(i)$

does give such a functor, which however is not in general an equivalence. Thus, even if C already has all filtered colimits, Ind(C) is a strictly larger category than C.

Objects in Ind(C) can be thought of as formal direct limits, so that some authors also denote such objects by

$\text{“}\underset{i\in I}{\underrightarrow{\lim }}\text{'' }F(i).$

This notation is due to Pierre Deligne.

Universal property of the ind-completion

The passage from a category C to Ind(C) amounts to freely adding filtered colimits to the category. This is why the construction is also referred to as the ind-completion of C. This is made precise by the following assertion: any functor $F:C\to D$ taking values in a category D that has all filtered colimits extends to a functor $Ind(C)\to D$ that is uniquely determined by the requirements that its value on C is the original functor F and such that it preserves all filtered colimits.

Basic properties of ind-categories

Compact objects

Essentially by design of the morphisms in Ind(C), any object X of C is compact when regarded as an object of Ind(C), i.e., the corepresentable functor

${\mathrm{Hom}}_{\mathrm{Ind}(C)}(X,-)$

preserves filtered colimits. This holds true no matter what C or the object X is, in contrast to the fact that X need not be compact in C. Conversely, any compact object in Ind(C) arises as the image of an object in X.

A category C is called compactly generated, if it is equivalent to $\mathrm{Ind}(C_0)$ for some small category $C_0$. The ind-completion of the category FinSet of finite sets is the category of all sets. Similarly, if C is the category of finitely generated groups, ind-C is equivalent to the category of all groups.

Recognizing ind-completions

These identifications rely on the following facts: as was mentioned above, any functor $F:C\to D$ taking values in a category D that has all filtered colimits, has an extension

$\tilde{F}:\mathrm{Ind}(C)\to D,$

that preserves filtered colimits. This extension is unique up to equivalence. First, this functor $\tilde{F}$ is essentially surjective if any object in D can be expressed as a filtered colimits of objects of the form $F(c)$ for appropriate objects c in C. Second, $\tilde{F}$ is fully faithful if and only if the original functor F is fully faithful and if F sends arbitrary objects in C to compact objects in D.

Applying these facts to, say, the inclusion functor

$F:\mathrm{FinSet}\subset \mathrm{Set},$

the equivalence

$\mathrm{Ind}(\mathrm{FinSet})\cong \mathrm{Set}$

expresses the fact that any set is the filtered colimit of finite sets (for example, any set is the union of its finite subsets, which is a filtered system) and moreover, that any finite set is compact when regarded as an object of Set.

The pro-completion

Like other categorical notions and constructions, the ind-completion admits a dual known as the pro-completion: the pro-completion Pro(C) can defined in terms of the ind-completion as

$\mathrm{Pro}(C):=\mathrm{Ind}(C^{op}{)}^{op}.$

(The original definition of pro-C is due to (Grothendieck 1960).)

Therefore, the objects of Pro(C) are inverse systems or pro-objects in C. By definition, these are direct system in the opposite category $C^{op}$ or, equivalently, functors

$F:I\to C$

from a small cofiltered category I.

Examples of pro-categories

While Pro(C) exists for any category C, several special cases are noteworthy because of connections to other mathematical notions.

• If C is the category of finite groups, then pro-C is equivalent to the category of profinite groups and continuous homomorphisms between them.
• The process of endowing a preordered set with its Alexandrov topology yields an equivalence of the pro-category of the category of finite preordered sets, $\mathrm{Pro}(\mathrm{FinPreord})$, with the category of spectral topological spaces and quasi-compact morphisms.
• Stone duality asserts that the pro-category $\mathrm{Pro}(\mathrm{FinSet})$ of the category of finite sets is equivalent to the category of Stone spaces.

The appearance of topological notions in these pro-categories can be traced to the equivalence, which is itself a special case of Stone duality,

${\mathrm{FinSet}}^{\text{op}}=\mathrm{FinBool}$

which sends a finite set to the power set (regarded as a finite Boolean algebra). The duality between pro- and ind-objects and known description of ind-completions also give rise to descriptions of certain opposite categories. For example, such considerations can be used to show that the opposite category of the category of vector spaces (over a fixed field) is equivalent to the category of linearly compact vector spaces and continuous linear maps between them.

Applications

Pro-completions are less prominent than ind-completions, but applications include shape theory. Pro-objects also arise via their connection to pro-representable functors, for example in Grothendieck's Galois theory, and also in Schlessinger's criterion in deformation theory.

Related notions

Tate objects are a mixture of ind- and pro-objects.

Infinity-categorical variants

The ind-completion (and, dually, the pro-completion) has been extended to ∞-categories by (Lurie 2009).

See also

• Direct limit – Special case of colimit in category theory
• Inverse limit – Construction in category theory
• completions in category theory

References

• Bergman; Hausknecht (1996), Cogroups and Co-rings in Categories of Associative Rings, Mathematical Surveys and Monographs, 45, doi:10.1090/surv/045, ISBN 9780821804957 
• Bourbaki, Nicolas (1968), Elements of mathematics. Theory of sets, Translated from the French, Paris: Hermann .
• Blom, Thomas; Moerdijk, Ieke (2023), "Simplicial model structures on pro-categories", Algebraic & Geometric Topology 23 (8): 3849–3908, doi:10.2140/agt.2023.23.3849 
• Grothendieck, Alexander (1960), "Technique de descente et théoèmes d'existence en géométrie algébriques. II. Le théorème d'existence en théorie formelle des modules" (in French), Séminaire Bourbaki : années 1958/59 - 1959/60, exposés 169-204, Sociétée mathématique de France, pp. 369–390, http://www.numdam.org/item/SB_1958-1960__5__369_0 
• Hazewinkel, Michiel, ed. (2001), "System (in a category)", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=S/s091930 
• Johnstone, Peter T. (1982), Stone Spaces, ISBN 0521337798 
• Lurie, Jacob (2009), Higher topos theory, Annals of Mathematics Studies, 170, Princeton University Press, ISBN 978-0-691-14049-0 
• Segal, Jack; Mardešić, Sibe (1982), Shape theory, North-Holland Mathematical Library, 26, Amsterdam: North-Holland, ISBN 978-0-444-86286-0, https://archive.org/details/shapetheoryinver0000mard