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Grothendieck topos

this entry should be merged with category of sheaves

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Topos Theory

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Definition

Classically, we have:

A Grothendieck topos 𝒯\mathcal{T} is a category that admits a geometric embedding

𝒯lexPSh(C) \mathcal{T} \stackrel{\stackrel{lex}{\leftarrow}}{\hookrightarrow} PSh(C)

in a presheaf category, i.e., a full and faithful functor that has a left exact left adjoint.

This is equivalently the category of sheaves (Set-valued presheaves satisfying the sheaf condition) over a small site.

Since smallness can be relative, we also have:

For a given fixed category of sets SS, a Grothendieck topos over SS is a category of sheaves (SS-valued presheaves satisfying the sheaf condition) over a site which is small relative to SS, that is a site internal to SS.

Note that a Grothendieck topos is a topos because (or if) SS is.

The site is not considered part of the structure; different sites may give rise to equivalent category of sheaves.

By the general theory of geometric morphisms, every Grothendieck topos sits inside a category of presheaves by a geometric embedding Sh(S)PSh(S)Sh(S) \hookrightarrow PSh(S).

Properties

General

Proposition

Every Grothendieck topos is a total category and a cototal category.

Proof

From the page total category, totality follows from the fact that a Grothendieck topos is

Dually, a Grothendieck topos is

Therefore a Grothendieck topos is also cototal.

Giraud's axiomatic characterization

Giraud characterized Grothendieck toposes as categories satisfying certain exactness and small completeness properties (where “small” is again relative to the given category of sets SS). The exactness properties are elementary (not depending on SS), and are satisfied in any elementary topos, or even a pretopos.

Giraud's theorem characterises a Grothendieck topos as follows:

  1. a locally small category with a small generating set,
  2. with all finite limits,
  3. with all small coproducts, which are disjoint, and pullback-stable,
  4. where all congruences have effective quotient objects, which are also pullback-stable.

These conditions are equivalent to

See the Elephant, theorem C.2.2.8. See also Wikipedia.

Sometimes (3,4) are combined and strengthened to the statement that the category has all small colimits, which are effective and pullback-stable. However, this is a mistake for two reasons: it is a significantly stronger axiomatisation (since without the small generating set, not every infinitary pretopos has this property), and it is not valid in weak foundations (while the definition given above is).

In weak foundations

We have two definitions of a Grothendieck topos:

  • the category of sheaves on some small site,
  • a category that satisfies Giraud's axioms (as listed above).

The theorem that these are equivalent can be proved in quite weak foundations, whether finitist, predicative, or constructive (or all three at once). Some hard-nosed predicativists (and even hard-nosed ZFC fundamentalists) may object to the language (on the ground that large things don't really exist), but they should accept the theorems when suitably phrased.

In predicative mathematics, however, we cannot prove that every Grothendieck topos is in fact a topos! In fact, it is immediate that the category of sets is a Grothendieck topos, but SetSet is an elementary topos if and only if power sets are small, which is precisely what predicativists doubt. One can use the term Grothendieck pretopos to avoid implying that we have an elementary topos. On the other hand, since Grothendeick toposes came first, perhaps it is the definition of ‘elementary topos’ that is too strong.

Similarly, in finitist mathematics, we cannot prove that every Grothendieck topos has a natural numbers object; while in weakly predicative constructive mathematics, we cannot prove that every Grothendieck topos is cartesian closed. In each case, once a property is accepted of SetSet (the axiom of infinity and small function sets, in these examples), it can be proved for all Grothendieck toposes.

Constructivism as such is irrelevant; even in classical mathematics, most Grothendieck toposes are not boolean. However, for an analogous result, try the theorem that the category of presheaves on a groupoid is boolean.

The theorem that every Grothendieck topos is cocomplete is a subtle point; it fails only in finitist predicative mathematics. (The key point in the proof is to generate the transitive closure *\sim^* of an binary relation \sim. One proof defines a *ba \sim^* b to mean that ax 0x n1ba \sim x_0 \sim \cdots \sim x_{n-1} \sim b for some nn, which is predicative but infinitary; another defines a *ba \sim^* b to mean that aba \sim' b for every transitive relation \sim' that contains \sim, which is finitary but impredicative.)

Locally presentable categories: Large categories whose objects arise from small generators under small relations.

(n,r)-categoriessatisfying Giraud's axiomsinclusion of left exaxt localizationsgenerated under colimits from small objectslocalization of free cocompletiongenerated under filtered colimits from small objects
(0,1)-category theory(0,1)-toposes\hookrightarrowalgebraic lattices\simeq Porst’s theoremsubobject lattices in accessible reflective subcategories of presheaf categories
category theorytoposes\hookrightarrowlocally presentable categories\simeq Adámek-Rosický’s theoremaccessible reflective subcategories of presheaf categories\hookrightarrowaccessible categories
model category theorymodel toposes\hookrightarrowcombinatorial model categories\simeq Dugger’s theoremleft Bousfield localization of global model structures on simplicial presheaves
(∞,1)-topos theory(∞,1)-toposes\hookrightarrowlocally presentable (∞,1)-categories\simeq
Simpson’s theorem
accessible reflective sub-(∞,1)-categories of (∞,1)-presheaf (∞,1)-categories\hookrightarrowaccessible (∞,1)-categories

References

A quick introduction of the basic facts of sheaf-topos theory is chapter I, “Background in topos theory” in

  • Ieke Moerdijk, Classifying Spaces and Classifying Topoi Lecture Notes in Mathematics 1616, Springer (1995)

A standard textbook on this case is

Grothendieck topoi appear around section III,4 there. A proof of Giraud’s theorem is in appendix A.

The proof of Giraud’s theorem for (∞,1)-topoi is section 6.1.5 of

Revised on May 20, 2014 15:47:53 by Toby Bartels (98.19.36.100)