nLab
test category

Contents

Idea

In the 1980-s, Grothendieck in Pursuing Stacks introduced test categories to make the variants of the homotopy theory based on the usage of combinatorial models with some kind of cell structure (e.g., simplicial sets, cubical sets and cellular sets) independent of a particular combinatorial model.

Definition

Given any small category 𝒞\mathcal{C}, one considers 𝒞\mathcal{C}-sets, hence presheaves on 𝒞\mathcal{C}, hence contravariant functors from 𝒞\mathcal{C} to SetSet.

Given an object cCc\in C, one considers the representable functor Hom 𝒞(,c)=:Δ cHom_{\mathcal{C}}(-,c)=:\Delta^c. If X:𝒞 opSetX:\mathcal{C}^{op} \to Set is a 𝒞\mathcal{C}-set, the elements of X(c)X(c) are called the cc-cells. By the Yoneda lemma, they correspond to the natural transformations Δ cX\Delta^c\to X.

Let the cell category of XX, denoted i 𝒞Xi_{\mathcal{C}} X, be the full subcategory of the overcategory 𝒞Set/X\mathcal{C}Set/X whose objects are the transformations of the form Δ cX\Delta^c\to X. The correspondence Xi 𝒞XX\mapsto i_{\mathcal{C}}X extends to a functor i 𝒞:𝒞SetCati_{\mathcal{C}}:\mathcal{C}Set\to Cat which has a right adjoint i 𝒞 *:Cat𝒞Seti_{\mathcal{C}}^*:Cat\to\mathcal{C}Set whose object part is given by the formula

i 𝒞 *(D)(c):=Hom 𝒞(𝒞/c,D). i_{\mathcal{C}}^*(D)(c):= Hom_{\mathcal{C}}(\mathcal{C}/c,D).

Denote the counit of the adjunction ϵ:i 𝒞i 𝒞 *Id Cat\epsilon : i_{\mathcal{C}}i_{\mathcal{C}}^*\to Id_{Cat}.

Two 𝒞\mathcal{C}-sets XX and YY are weakly equivalent if there is a map f:XYf:X\to Y inducing an equivalence f *:i 𝒞Xi 𝒞Yf_* : i_{\mathcal{C}} X\to i_{\mathcal{C}} Y of their cell categories, i.e., the induced map of nerves (“classifying spaces”) B(i 𝒞X)B(i 𝒞Y)B(i_{\mathcal{C}} X)\to B(i_{\mathcal{C}} Y) is a weak equivalence of simplicial sets. Functor i 𝒞:𝒞SetCati_{\mathcal{C}}:\mathcal{C}Set\to Cat induces a functor i 𝒞*:Ho(𝒞Set)Ho(Cat)i_{\mathcal{C}*}:Ho(\mathcal{C}Set)\to Ho(Cat) of the homotopy categories.

A weak test category is a small category 𝒞\mathcal{C} such that, for any category DD in CatCat, the component of the counit ϵ D:i 𝒞i 𝒞 *DD\epsilon_D : i_{\mathcal{C}}i_{\mathcal{C}}^* D \to D is an equivalence of categories.

A test category is any small category 𝒜\mathcal{A} such that

A strict test category is a test category 𝒜\mathcal{A} such that

  • i 𝒞:𝒞SetCati_{\mathcal{C}} : \mathcal{C}Set \to Cat preserves finite products up to weak equivalence,

or equivalently, such that

  • the induced functor i 𝒞*:Ho(𝒞Set)Ho(Cat)i_{\mathcal{C}*}:Ho(\mathcal{C}Set)\to Ho(Cat) preserves finite products.

Then one proceeds with 𝒜\mathcal{A}-sets.

If 𝒜\mathcal{A} is a test category and 𝒞\mathcal{C} any small category whose classifying space is contractible (which may or may not be a test category itself), then their cartesian product 𝒜×𝒞\mathcal{A}\times\mathcal{C} is a test category.

Properties

Homotopy category

The homotopy category of a category of presheaves over a test category, as a category with weak equivalences is equivalent to the standard homotopy category of homotopy theory: that of the category of simplicial sets/topological spaces with weak equivalences being weak homotopy equivalences.

In other words, presheaves over a test category are models for homotopy types of ∞-groupoids.

Model category structure

The presheaf category over a test category with the above weak equivalences admits a model category structure: the model structure on presheaves over a test category. This is due to (Cisinski) with further developments due to (Jardine).

Examples

Apart from the archeytpical example of the simplex category we have the following

References

The notion of test category was introduced in

Various conjectures made their are proven in

which moreover develops the main toolset and establishes the model structure on presheaves over a test category.

General surveys include

  • Georges Maltsiniotis, La théorie de l’homotopie de Grothendieck, Astérisque, 301, pp. 1-140, (2005) (see html)

  • Rick Jardine, Categorical homotopy theory, Homot. Homol. Appl. 8 (1), 2006, pp.71–144, (HHA, pdf)

That the cube category is a test category is asserted without proof in (Grothendieck). A proof is spelled out in (Cisinski)

That it is not a strict test category was known in

  • Dan Kan, Abstract homotopy. I , Proc. Nat. Acad. Sci. U.S.A. 41 (1955), 1092–1096.

and led to the preferences of simplicial sets over cubical sets.

That the category of cubes equipped with connection on a cubical set form a strict test category is shown in

  • Georges Maltsiniotis, La catégorie cubique avec connexions est une catégorie test stricte . (French. English summary) Homology, Homotopy Appl. 11 (2009), no. 2, 309–326. (web)

The test category nature of the groupoidal Theta category is discussed in

Revised on July 19, 2013 02:28:31 by Matan P (176.31.26.4)