nLab pretopos

Definitions

General

A pretopos is a category which is both (Barr-)exact and extensive. (See familial regularity and exactness for why extensivity and exactness deserve to be considered together.) This implies that it is a coherent category.

Pretoposes are suitable as frameworks for finitist predicative mathematics, respectively with coherent logic.

Frequently one is especially interested in pretoposes having additional properties, such as:

• A Heyting pretopos is a pretopos which is also a Heyting category; a Boolean pretopos is a pretopos which is also a Boolean category. These are suitable as frameworks for finitist predicative mathematics, respectively with intuitionistic or classical logic.

• A pretopos with choice is a pretopos that satisfies the axiom of choice (that every epimorphism is split). Every pretopos with choice is automatically boolean.

• A Π-pretopos is a pretopos which is also a locally cartesian closed category. (A $\Pi$-pretopos is automatically a Heyting pretopos.) These are suitable as frameworks for finitist constructive mathematics which is ‘weakly predicative’.

• An arithmetic pretopos is a pretopos which has (locally) a parameterised natural numbers object. Arithmetic pretoposes are suitable as frameworks for non-finitist but strongly predicative mathematics with coherent logic, while Heyting arithmetic pretoposes and Boolean arithmetic pretoposes are suitable as frameworks for non-finitist but strongly predicative mathematics with intuitionistic and classical logic, respectively.

• A $W$-pretopos is a pretopos which has (locally) W-types (initial algebras for polynomial endofunctors), most famously a natural numbers object. (Here, we take any exponentiable morphism to define a polynomial endofunctor. This paragraph is original to this page and not in the literature; it may not be the best definition.)

• An arithmetic $\Pi$-pretopos is a pretopos that is both a $\Pi$-pretopos and an arithmetic pretopos; it is sufficient that the $\Pi$-pretopos have a natural numbers object, and so could be called $\Pi$-pretopos with NNO. These are suitable as frameworks for weakly predicative constructive mathematics that is not finitist.

• A ΠW-pretopos is a pretopos that is both a $\Pi$-pretopos and a $W$-pretopos. (Now every morphism defines a polynomial endofunctor, since every morphism is exponentiable.)

• A predicative topos is a ΠW-pretopos that satisfies the axiom of multiple choice. (This now differs from a $W$-topos below mainly in still lacking power objects, hence still a context for weakly predicative constructive mathematics.)

• A topos is a pretopos that has power objects. A topos is automatically a $\Pi$-pretopos; conversely, a $\Pi$-pretopos is a topos iff it has a subobject classifier, and a Boolean $\Pi$-pretopos is always a topos. Toposes and Boolean toposes are suitable as frameworks for finitist (but otherwise impredicative) mathematics, with intuitionistic and classical logic respectively.

• A $W$-topos is of course a topos that is a $W$-pretopos; it is sufficient that the topos have a natural numbers object (see van den Berg, Moerdijk 2008), so this is often called a topos with NNO. These are suitable as frameworks for (non-predicative, non-finitist) constructive mathematics, while Boolean $W$-toposes are suitable as a framework for classical mathematics without the axiom of choice.

• A topos with choice is a topos that satisfies the axiom of choice (that every epimorphism is split). Every topos with choice is automatically boolean, so $W$-toposes with choice are suitable as a framework for full classical mathematics. (In fact, a well-pointed $W$-topos with choice is precisely a model of ETCS.)

Infinitary pretoposes

An infinitary pretopos is an infinitary coherent category which is both infinitary extensive and exact. Giraud's theorem says that infinitary pretoposes with small generating sets are the same as Grothendieck toposes, and in particular are toposes (although this last result is not valid in predicative mathematics).

An infinitary pretopos (perhaps with well-poweredness included) is sometimes called an “$\infty$-pretopos” (e.g. in the Elephant), but we avoid that term because of potential confusion with the notion of (∞,1)-pretopos.

Infinitary pretoposes are precisely the algebras for the free cocompletion pseudomonad on $Lex$, the 2-category of finitely complete categories. See GL12.

Examples

• Of course, any topos is a pretopos, so any example there is an example of a pretopos.

• An example of a pretopos that is not a topos is the category of compact Hausdorff spaces (MarraReggio).

• An example of a Boolean pretopos that is not cartesian closed is the category of finite or countably infinite sets (cf. Elephant p.54).

• The category of decidable objects of a topos is a Boolean pretopos.

• The exact completion of the category Ho(Top) of topological spaces and homotopy classes of continuous maps is a pretopos.

• In dependent type theory, a univalent universe where every type in the universe satisfies axiom K and is closed under the empty type, unit type, sum types, dependent sum types, product types, propositional truncations, and quotient sets is a pretopos.

  • the universe becomes an arithmetic pretopos if it is closed under a parameterized natural numbers type
  • the universe becomes a Heyting pretopos if the universe is closed under dependent product types for families of mere propositions, or equivalently, if it is closed under types of embeddings
  • the universe becomes a Boolean pretopos if it is a Heyting pretopos and additionally every proposition satisfies excluded middle
  • the universe becomes a Π-pretopos if it is closed under all dependent product types, or equivalently, if the universe is closed under function types
  • the universe becomes an arithmetic Π-pretopos if it is a Π-pretopos and is closed under a natural numbers type
  • the universe becomes an elementary topos if it is a Π-pretopos and has a type of all propositions
  • the universe becomes a Boolean topos if it is a Boolean Π-pretopos

Properties

Internal logic and mathematics in pretopoi

Like any coherent (or Heyting) category, a (Heyting) pretopos has an internal logic. Extensivity and exactness make a Heyting pretopos a very set-like category. One can say imprecisely that it has “all the good first-order properties of a topos”, meaning not that it has those properties that can be expressed in elementary terms (which is false) but that it has those properties that (unlike exponential and power objects) correspond to first-order reasoning in ordinary mathematics. Therefore, pretoposes (especially Heyting, $\Pi$, and/or $W$ ones) are related to predicative constructive mathematics in a way similar to how toposes are related to non-predicative constructive mathematics.

Colimits

A pretopos is necessarily balanced, but while it has coproducts and coequalizers of equivalence relations, it need not have all finite colimits. However, if it has countable pullback-stable unions of subobjects (what the Elephant calls a $\sigma$-pretopos; see infinitary coherent category), then any internal binary relation generates an equivalence relation and therefore has a quotient, so we can construct arbitrary coequalizers and thus arbitrary finite colimits. And we can perform an “internal” version of this argument in a $\Pi$-pretopos with a NNO, such as a $\Pi$-$W$-pretopos.

The precanonical topology

A pretopos, being a coherent category, admits a subcanonical Grothendieck topology called the coherent topology. In a pretopos, this topology is generated by finite jointly epimorphic families. Since the canonical topology on a Grothendieck topos consists of all jointly epimorphic families, the coherent topology on a pretopos is sometimes called the precanonical topology.

The codomain fibration of a pretopos is always a stack for its precanonical topology. Being a pretopos is stronger than necessary for this condition to hold in a coherent category, however; see coherent category for the necessary and sufficient conditions.

Related concepts

• Π-pretopos, ΠW-pretopos, list-arithmetic pretopos

• 2-pretopos

• (∞,1)-pretopos

• conceptual completeness

References

• Benno van den Berg, Ieke Moerdijk (2008-09-25); $W$-types in sheaves; vdBM_Wtypes.ps/pdf

• Vincenzo Marra?, Luca Reggio?, A characterisation of the category of compact Hausdorff spaces, Theory Appl. Categ. 35 (2020), Paper No. 51, 1871–1906. pdf, arXiv:1808.09738.

• Paul Taylor, MathOverflow answer.

• Marino Gran, Enrico Vitale, On the exact completion of the homotopy category, Cahiers de Topologie et Géométrie Différentielle Catégoriques 39 4 (1998) 287-297 [numdam:CTGDC_1998__39_4_287_0]

• Richard Garner and Stephen Lack, Lex colimits, JPAA 216.6, 2012.