model category, model $\infty$-category
Definitions
Morphisms
Universal constructions
Refinements
Producing new model structures
Presentation of $(\infty,1)$-categories
Model structures
for $\infty$-groupoids
on chain complexes/model structure on cosimplicial abelian groups
related by the Dold-Kan correspondence
for equivariant $\infty$-groupoids
for rational $\infty$-groupoids
for rational equivariant $\infty$-groupoids
for $n$-groupoids
for $\infty$-groups
for $\infty$-algebras
general $\infty$-algebras
specific $\infty$-algebras
for stable/spectrum objects
for $(\infty,1)$-categories
for stable $(\infty,1)$-categories
for $(\infty,1)$-operads
for $(n,r)$-categories
for $(\infty,1)$-sheaves / $\infty$-stacks
homotopy theory, (∞,1)-category theory, homotopy type theory
flavors: stable, equivariant, rational, p-adic, proper, geometric, cohesive, directed…
models: topological, simplicial, localic, …
see also algebraic topology
Introductions
Definitions
Paths and cylinders
Homotopy groups
Basic facts
Theorems
By regarding a simplicial set as an object in the classical model structure on simplicial sets, one effectively identifies it (up to weak equivalence) with the ∞-groupoid that it presents under Kan fibrant replacement. If the original simplicial set is the nerve of a category, then the corresponding Kan fibrant replacement is something like the $\infty$-groupoidification of that category: see geometric realization of categories.
This way each ordinary category models an ∞-groupoid. The Thomason model category structure on Cat exhibits this: in this model category a morphism between two categories is a weak equivalence precisely if it induces a weak equivalence of the corresponding $\infty$-groupoids.
It turns out the Thomason model structure on Cat is Quillen equivalent to the classical model structure on simplicial sets.
This is remarkable, in that it says that every homotopy type, i. e. every equivalence class of $\infty$-groupoids, is obtained by $\infty$-groupoidifying just categories.
In fact, every cofibrant object in the Thomason model structure structure is a poset. Since every object in a model category is weakly equivalent to a cofibrant one, this means that even the nerves of just posets are sufficient to model all homotopy types.
This is a rather curious aspect of the Thomason model on Cat: it does not really have anything intrinsically to do with categories, but rather uses these as a way to present ∞-groupoids. In particular, the class of weak equivalences is much larger than just the equivalences of categories. There is a different model structure on Cat in which the weak equivalences are precisely the “true” weak equivalences of categories (not of anything constructed from them). This is called the canonical model structure on Cat.
Recall the subdivision functor $Sd$ and its right adjoint Ex-functor.
Let $C$ and $D$ be small categories. A functor $f \colon C \to D$ is called :
a Thomason fibration iff $Ex^2 N(f)$ is a Kan fibration,
Thomason weak equivalence iff its nerve $N(f)$ is a weak homotopy equivalence,
Thomason-cofibration iff it has the left lifting property against the morphisms that are both Thomason-fibrations as well as Thomason-equivalences.
Equipped with these classes of morphisms, the 1-category of small categories is a proper model category.
Every cofibrant object in the Thomas model structure (Def. ) is a poset (regarded as a small category).
The Thomason model structure on Cat is Quillen equivalent to the classical model structure on simplicial sets by the adjunction
We can assert more: this is also an adjoint weak equivalence. These are relative functors between the relative categories $(sSet, Kan)$ and $(Cat, Thomason)$, and both the adjunction unit and counit are natural weak equivalences.
The localization $L : Cat \to \infty Gpd$ sending a category to its homotopy type has a direct interpretation in terms of ∞-category theory:
$L$ is the ∞-groupoidification functor functor $C \mapsto C[C^{-1}]$.
Let $f : C \to Cat$ be a functor. Its homotopy colimit in the Thomason model structure can be computed using the Grothendieck construction:
By Hirschhorn proposition 18.1.6, the nerve of any category is the homotopy colimit of the constant point-valued diagram:
For any category $C$, $N(C) \simeq hocolim_{c \in C} \Delta^0$
In fact, when Hirschhorn’s definition of the homotopy colimit for functors valued in simplicial model categories:
we get an isomorphism $N(C)^{op} \cong hocolim(1)$.
The original reference:
A correction to this article was made in
Review and generalization to $n$-fold categories:
Restriction of the Thomason model structure to the category of just posets:
and to the category of just (small) acyclic categories:
Discussion of fibrant objects in the Thomason model structure:
Lennart Meier, Viktoriya Ozornova, Fibrancy of partial model categories, Homology, Homotopy and Applications, Volume 17 (2015) Number 2. (arXiv:1408.2743, doi:10.4310/HHA.2015.v17.n2.a5)
Lennart Meier, Fibrancy of (Relative) Categories, talk at Young Topologists Meeting 2014 (slides pdf)
Discussion of cofibrant objects (Thomason-cofibrant posets):
Roman Bruckner, Christoph Pegel, Cofibrant objects in the Thomason Model Structure, [arXiv:1603.05448]
Roman Bruckner, Abstract Homotopy Theory and the Thomason Model Structure, PhD thesis, Bremen 2016 [gbv:46-00105527-15, pdf]
On the analog of the Thomason model structure for the stable case – an equivalence between connective stable homotopy theory and nerves of symmetric monoidal categories:
Some other references
Last revised on May 1, 2023 at 18:15:30. See the history of this page for a list of all contributions to it.