nLab
pro-object

Contents

Idea

A pro-object of a category CC is a “formal cofiltered limit” of objects of CC.

The category of pro-objects of CC is written propro-CC. Such a category is sometimes called a pro-category, but notice that that is not the same thing as a pro-object in Cat.

“Pro” is short for “projective”. ( Projective limit is an older term for limit.) It is in contrast to “ind” in the dual notion of ind-object, standing for “inductive”, (and corresponding to inductive limit, the old term for colimit). In some (often older) sources, the term ‘projective system’ is used more or less synonymously for pro-object.

The definition of arrows in the category of pro-objects in 𝒞\mathcal{C} is perhaps more intuitive in the dual case of ind-objects (pro-objects in C opC^{op}), where it can be seen as stipulating that the objects of CC are finitely presentable in indind-CC.

Details

Definition

A pro-object in a category 𝒞\mathcal{C} is a functor F:𝒟𝒞F: \mathcal{D} \rightarrow \mathcal{C} for some small cofiltered category 𝒟\mathcal{D}.

Pro-objects in a category 𝒞\mathcal{C} assemble into a category as follows.

Definition

Let 𝒞\mathcal{C} be a category. The category of pro-objects in 𝒞\mathcal{C} is the category defined as follows.

  1. The objects are pro-objects in 𝒞\mathcal{C}.
  2. The set of arrows from a pro-object F:𝒟𝒞F: \mathcal{D} \rightarrow \mathcal{C} to a pro-object G:𝒞G: \mathcal{E} \rightarrow \mathcal{C} is the limit of the functor 𝒟 op×Set\mathcal{D}^{op} \times \mathcal{E} \rightarrow \mathsf{Set} given by Hom 𝒞(F(),G())Hom_{\mathcal{C}}\left(F(-), G(-)\right).
  3. Composition of arrows arises, given pro-objects F:𝒟 0𝒞F: \mathcal{D}_{0} \rightarrow \mathcal{C}, G:𝒟 1𝒞G: \mathcal{D}_{1} \rightarrow \mathcal{C}, and H:𝒟 2𝒞H: \mathcal{D}_{2} \rightarrow \mathcal{C} of 𝒞\mathcal{C}, by applying the limit functor for diagrams 𝒟 op×Set\mathcal{D}^{op} \times \mathcal{E} \rightarrow \mathsf{Set} to the natural transformation of functors Hom 𝒞(F(),G())×Hom 𝒞(G(),H())Hom 𝒞(F(),H())Hom_{\mathcal{C}}\left(F(-), G(-)\right) \times Hom_{\mathcal{C}}\left(G(-), H(-)\right) \rightarrow Hom_{\mathcal{C}}\left( F(-), H(-) \right) given by composition in 𝒞\mathcal{C}.
  4. The identity arrow on a pro-object F:𝒟𝒞F: \mathcal{D} \rightarrow \mathcal{C} arises, using the universal property of a limit, from the identity arrow Hom 𝒞(F(c),F(c))Hom_{\mathcal{C}}\left(F(c), F(c)\right) for every object cc of CC.

That the associativity and identity axioms hold follows immediately from the fact that they hold in 𝒞\mathcal{C}.

Notation

We denote the category of Definition by pro-𝒞\mathcal{C}.

Remark

For brevity, we sometimes write the hom set between F:𝒟𝒞F: \mathcal{D} \to \mathcal{C} and G:𝒞G: \mathcal{E} \to \mathcal{C} as

limeOb()colimdOb(𝒟)𝒞(Fd,Ge), \underset{e \in Ob(\mathcal{E})}{lim}\, \underset{d\in Ob(\mathcal{D})}{colim} \mathcal{C}(F d, G e),

where the limit and colimit is taken in the category Set of sets.

Remark

We can give an explicit description of the arrows of pro-𝒞\mathcal{C} as follows. First, for any object ee of \mathcal{E}, we introduce a relation \sim on arrows with target G(e)G(e) which identifies an arrow f:F(d)G(e)f: F(d) \rightarrow G(e) with an arrow f:F(d)G(e)f': F(d') \rightarrow G(e) for objects dd and dd' of 𝒟\mathcal{D} and an object ee of \mathcal{E}, if there is an object dd'' of 𝒟\mathcal{D}, an arrow g:ddg: d'' \rightarrow d of 𝒟\mathcal{D}, and an arrow g:ddg': d'' \rightarrow d' of 𝒟\mathcal{D}, such that fF(g)=fF(g)f \circ F(g) = f' \circ F(g').

This relation \sim is in fact an equivalence relation. Symmetry is obvious. Reflexivity is immediately demonstrated using the identity arrows of 𝒟\mathcal{D}. Transitivity would not hold for an arbitrary category, but follows from the assumption that 𝒟\mathcal{D} is cofiltered. Indeed, suppose that we have a zig-zag in 𝒟\mathcal{D} as follows.

The fact that 𝒟\mathcal{D} is cofiltered ensures that there is an object dd'' of 𝒟\mathcal{D} fitting into the following diagram.

Suppose that we have arrows f 0:F(d 0)G(e)f_{0} : F(d_{0}) \rightarrow G(e), f 1:F(d 1)G(e)f_{1}: F(d_{1}) \rightarrow G(e), and f 2:F(d 2)G(e)f_{2}: F(d_{2}) \rightarrow G(e) such that g 0g_{0} and g 1g_{1} exhibit that f 0f 1f_{0} \sim f_{1}, and such that g 2g_{2} and g 3g_{3} exhibit that f 1f 2f_{1} \sim f_{2}. Then

f 0F(g 0g 0) =f 0F(g 0)F(g 0) =f 1F(g 1)F(g 0) =f 1F(g 2)F(g 1) =f 2F(g 3)F(g 1) =f 2F(g 3g 1). \begin{aligned} f_{0} \circ F(g_{0} \circ g'_{0}) &= f_{0} \circ F(g_{0}) \circ F(g'_{0}) \\ &= f_{1} \circ F(g_{1}) \circ F(g'_{0}) \\ &= f_{1} \circ F(g_{2}) \circ F(g'_{1}) \\ &= f_{2} \circ F(g_{3}) \circ F(g'_{1}) \\ &= f_{2} \circ F(g_{3} \circ g'_{1}). \end{aligned}

This exhibits that f 0f 2f_{0} \sim f_{2}, as required.

With this equivalence relation \sim to hand, we can give our explicit description of the arrows of pro-𝒞\mathcal{C}: an arrow of pro-𝒞\mathcal{C} from a pro-object F:𝒟𝒞F: \mathcal{D} \rightarrow \mathcal{C} to a pro-object G:𝒞G: \mathcal{E} \rightarrow \mathcal{C} can be taken to be a set {f e:F(d e)G(e)}\left\{ f_{e} : F\left(d_{e}\right) \rightarrow G(e) \right\} of arrows of 𝒞\mathcal{C}, one for every object ee of \mathcal{E}, such that, for every arrow g:eeg: e \rightarrow e' of EE, G(g)f eG(g)f eG(g) \circ f_{e} \sim G(g) \circ f_{e'}.

In other words: a set {f e:F(d e)G(e)}\left\{ f_{e} : F\left(d_{e}\right) \rightarrow G(e) \right\} of arrows of 𝒞\mathcal{C}, one for every object ee of \mathcal{E}, such that, for every arrow g:eeg: e \rightarrow e' of EE, there is an object dd of 𝒟\mathcal{D}, an arrow g e:dd eg_{e} : d \rightarrow d_{e} of 𝒟\mathcal{D}, and an arrow g e:dd eg_{e'}: d \rightarrow d_{e'} of 𝒟\mathcal{D} such that G(g)f eF(g e)=G(g)f eF(g e)G(g) \circ f_{e} \circ F(g_{e}) = G(g) \circ f_{e'} \circ F(g_{e'}).

Two such sets {f e} eOb()\left\{ f_{e} \right\}_{e \in Ob(\mathcal{E})} and {f e} eOb()\left\{ f'_{e} \right\}_{e \in Ob(\mathcal{E})} are equal, i.e. define the same arrow from FF to GG, if f ef ef_{e} \sim f'_{e} for every object ee of \mathcal{E}.

Alternative points of view

Via filtered limits of presheaves

Another, equivalent, definition is to let propro-CC be the full subcategory of the opposite functor category/presheaf category [C,Set] op[C,Set]^{op} determined by those functors which are cofiltered limits of representables. This is reasonable since the copresheaf category [C,Set] op[C,Set]^{op} is the free completion of CC, so propro-CC is the “free completion of CC under cofiltered limits.” See also at pro-representable functor.

The equivalence with the previous definition is seen as follows. To a functor F:ICF: I \to C, compose with the co-Yoneda embedding C[C,Set] opC \to [C,Set]^{op} to obtain a functor F˜:I[C,Set] op\tilde F: I \to [C, Set]^{op}, and then take |F|=limF˜[C,Set] op|F| = lim \tilde F \in [C,Set]^\mathrm{op}. Explicitly, |F|(c)=colimF˜ op|F|(c) = colim \tilde F^{op}. This yields a functor Pro(C)[C,Set] opPro(C) \to [C,Set]^{op}, and its essential image manifestly consists of the functors which are cofiltered limits of the duals of representables. To see that this functor is fully faithful, we compute, for F:ICF: I \to C and G:JCG: J \to C:

Hom(|F|,|G|)=Nat(colimG˜ op,colimF˜ op) Hom(|F|,|G|) = Nat(colim \tilde G^\mathrm{op}, colim \tilde F^\mathrm{op})

=lim J opNat(G˜ op,colimF˜ op)= lim_{J^{op}} Nat(\tilde G^\mathrm{op}, colim \tilde F^\mathrm{op})

=lim J opcolim I opNat(G˜ op,F˜ op)= lim_{J^{op}} colim_{I^\mathrm{op}} Nat(\tilde G^\mathrm{op}, \tilde F^\mathrm{op})

=lim J opcolim I opHom 𝒞(F,G)= lim_{J^{op}}colim_{I^\mathrm{op}} Hom_\mathcal{C}(F,G)

as in Pro(C)Pro(C). Here we have used the definition of a colimit, the fact that representables are compact objects (this follows from the fact that colimits are computed “levelwise” in a functor category), and the Yoneda lemma.

As formal duals of ind-objects

Remark

The category of pro-objects in 𝒞\mathcal{C} is the opposite category of that of ind-objects in the opposite catgegory of 𝒞\mathcal{C}:

Pro(𝒞)(Ind(𝒞 op)) op. Pro(\mathcal{C}) \simeq (Ind(\mathcal{C}^{op}))^{op} \,.

(e.g. Kashiwara-Schapira 06, p. 138)

Characterisations

In some cases, pro-objects in a category 𝒞\mathcal{C} can be viewed as actual limits in a certain category. We prove here some results of this kind.

Proposition

Let 𝒞\mathcal{C} be a category, and let 𝒜\mathcal{A} be a category with cofiltered limits. Suppose that there exists a fully faithful functor R:𝒞𝒜R: \mathcal{C} \rightarrow \mathcal{A}. Then pro𝒞pro-\mathcal{C} is equivalent to the full subcategory pro 𝒜𝒞pro^{\mathcal{A}}-\mathcal{C} of 𝒜\mathcal{A} whose objects are isomorphic to lim(RD)lim(R \circ D) for some diagram DD in 𝒞\mathcal{C} (this can be given a constructive interpretation according to whether the reader prefers to avoid the axiom of choice in the proof), where limlim is the limit functor for diagrams in 𝒜\mathcal{A}.

An equivalence of categories is given by the functor pro𝒞pro 𝒜𝒞pro-\mathcal{C} \rightarrow pro^{\mathcal{A}}-\mathcal{C} which on objects sends a pro-object d:𝒟𝒞d: \mathcal{D} \rightarrow \mathcal{C} to the limit of the functor Rd:𝒟𝒜R \circ d: \mathcal{D} \rightarrow \mathcal{A}, and on arrows sends the limit of the diagram in the category of sets

Hom 𝒞(d 1(),d 2()) Hom_{\mathcal{C}}\left(d_{1}(-), d_{2}(-) \right)

to the limit of the diagram in the category of sets

Hom 𝒜(Rd 1(),Rd 2()), Hom_{\mathcal{A}}\left(R \circ d_{1}(-), R \circ d_{2}(-) \right),

using the natural transformation arising from applying RR, and then applies the natural isomorphism between the limit of the above diagram and the diagram in the category of sets

Hom 𝒜(lim(Rd 1()),lim(Rd 2()) Hom_{\mathcal{A}}\left(lim \left(R \circ d_{1}(-) \right), lim \left( R \circ d_{2}(-) \right) \right.

Proof

Since RR is fully faithful, the natural transformation from the diagram

Hom 𝒞(d 1(),d 2()) Hom_{\mathcal{C}}\left(d_{1}(-), d_{2}(-) \right)

to the diagram

Hom 𝒜(Rd 1(),Rd 2()), Hom_{\mathcal{A}}\left(R \circ d_{1}(-), R \circ d_{2}(-) \right),

is in fact a natural isomorphism. Since, by definition, the objects of pro 𝒜𝒞pro^{\mathcal{A}}-\mathcal{C} are exactly those isomorphic to those arising by applying the functor pro𝒞pro 𝒜𝒞pro-\mathcal{C} \rightarrow pro^{\mathcal{A}}-\mathcal{C} to the objects of pro-𝒞\mathcal{C}, it follows immediately that this functor is one half of an equivalence of categories.

Example

Let 𝒞\mathcal{C} be the category Grp of groups, and let 𝒜\mathcal{A} be the category TopGrp\mathsf{Top-Grp} of topological groups. The fully faithful functor SetTop\mathsf{Set} \rightarrow \mathsf{Top} sending a set to the discrete topological space on this set gives rise to a fully faithful functor GrpTopGrp\mathsf{Grp} \rightarrow \mathsf{Top-Grp}. Then Proposition implies that the category pro-FinGrp\mathsf{FinGrp} of pro-objects in FinGrp\mathsf{FinGrp}, that is to say of profinite groups, is equivalent to the full sub-category of topological groups whose objects are obtained as a cofiltered limit of finite groups (viewed as topological groups via the discrete topology).

Remark

Though it is less well-known, one can in Example evidently replace Top\mathsf{Top} with any category 𝒜\mathcal{A} for which there is a fully faithful functor Set𝒜\mathsf{Set} \rightarrow \mathcal{A} which preserves finite products. See discrete object for one general setting in which such a functor exists. For example, one can take 𝒟\mathcal{D} to be the category sSet of simplicial sets.

Remark

Both Example and Remark generalise from Grp\mathsf{Grp} to any finite product theory, that is to say to the category of models of a finite product sketch. They generalise further to any finite limit theory, that is to say to the category of models of a finite limit sketch, if the functor Set𝒜\mathsf{Set} \rightarrow \mathcal{A} moreover preserves finite limits.

Examples

Applications

Étale homotopy theory.

Procategories were used by Artin and Mazur in their work on étale homotopy theory. They associated to a scheme a ‘pro-homotopy type’. (This is discussed briefly at étale homotopy.) The important thing to note is that this was a pro-object in the homotopy category of simplicial sets, i.e., in the pro-homotopy category. Friedlander rigidified their construction to get an object in the pro-category of simplicial sets, and this opened the door to use of ‘homotopy pro-categories’.

Shape theory.

The form of shape theory developed by Mardešić and Segal, at about the same time as the work in algebraic geometry, again used the pro-homotopy category. Strong shape, developed by Edwards and Hastings, and also Porter and also in further work by Mardešić and Segal, used various forms of rigidification to get to the pro-category of spaces, or of simplicial sets. There methods of model category theory could be used.

References

  • Alexander Grothendieck, Techniques de déscente et théorèmes d’existence en géométrie algébrique, II: le théorème d’existence en théorie formelle des modules, Seminaire Bourbaki 195, 1960, (pdf).

  • (SGA4-1) Alexander Grothendieck, Jean-Louis Verdier, Préfaisceaux, Exp. 1 (retyped pdf) in Théorie des topos et cohomologie étale des schémas. Tome 1: Théorie des topos, Séminaire de Géométrie Algébrique du Bois-Marie 1963–1964 (SGA 4). Dirigé par M. Artin, A. Grothendieck, et J. L. Verdier. Avec la collaboration de N. Bourbaki, P. Deligne et B. Saint-Donat. Lecture Notes in Mathematics 269, Springer 1972. pdf of SGA 4, Tome 1

  • Michael Artin, Barry Mazur, appendix of Étale homotopy theory, Lecture Notes in Maths. 100, Springer-Verlag, Berlin 1969.

  • Jean-Marc Cordier, and Tim Porter, Shape Theory, categorical methods of approximation, Dover (2008) (This is a reprint of the 1989 edition without amendments.)

  • Masaki Kashiwara, and Pierre Schapira, section 6 of Categories and Sheaves, Grundlehren der mathematischen Wissenschaften 332 (2006)

  • Peter Johnstone, section VI.1 of Stone Spaces

  • Dan Isaksen, Calculating limits and colimits in pro-categories, Fund. Math. 175 (2002), no. 2, 175–194.

  • S. Mardešić, J. Segal, Shape theory, North Holland 1982

  • Jean-Louis Verdier, Equivalence essentielle des systèmes projectifs, C. R.A.S. Paris261 (1965), 4950 - 4953.

  • John Duskin, Pro-objects (after Verdier), Sém. Heidelberg- Strasbourg1966 -67, Exposé 6, I.R.M.A.Strasbourg.

  • A. Deleanu, P. Hilton, Borsuk shape and Grothendieck categories of pro-objects, Math. Proc. Camb. Phil. Soc.79-3 (1976), 473-482 MR400220

  • Walter Tholen, Pro-categories and multiadjoint functors, Canadian J. Math. 36:1 (1984) 144-155 doi

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