Definitions
Transfors between 2-categories
Morphisms in 2-categories
Structures in 2-categories
Limits in 2-categories
Structures on 2-categories
The notion of a 2-category generalizes that of category: a 2-category is a higher category, where on top of the objects and morphisms, there are also 2-morphisms. In old texts, strict 2-categories are occasionally called hypercategories.
A 2-category consists of
1-morphisms between objects;
2-morphisms between morphisms.
The morphisms can be composed along the objects, while the 2-morphisms can be composed in two different directions: along objects – called horizontal composition – and along morphisms – called vertical composition. The composition of morphisms is allowed to be associative only up to coherent associator 2-morphisms.
2-categories are also a horizontal categorification of monoidal categories: they are like monoidal categories with many objects.
2-categories provide the context for discussing
monads.
The concept of 2-category generalizes further in higher category theory to n-categories, which have k-morphisms for all .
2-categories form a 3-category, 2Cat.
The easiest definition of 2-category is that it is a category enriched over the cartesian monoidal category Cat. Thus it has a collection of objects,. and for each pair of objects a category . The objects of these hom-categories are the morphisms, and the morphisms of these hom-categories are the 2-morphisms. This produces the classical notion of strict 2-category.
For some purposes, strict 2-categories are too strict: one would like to allow composition of morphisms to be associative and unital only up to coherent invertible 2-morphisms. A direct generalization of the above “enriched” definition produces the classical notion of bicategory.
One can also obtain notions of 2-category by specialization from the case of higher categories. Specifically, if we fix a meaning of -category, however weak or strict we wish, then we can define a -category to be an -category such that every 3-morphism is an equivalence, and all parallel pairs of -morphisms are equivalent for . It follows that, up to equivalence, there is no point in mentioning anything beyond -morphisms, except whether two given parallel -morphisms are equivalent. In some models of -categories, it is possible to make this precise by demanding that all parallel pairs of -morphisms are actually equal for , producing a simpler notion of 2-category in which we can speak about equality of 2-morphisms instead of equivalence. (This is the case for both strict -categories and bicategories.)
All of the above definitions produce “equivalent” theories of 2-category, although in some cases (such as the fact that every bicategory is equivalent to a strict 2-category) this requires some work to prove. On the nLab, we often use the word “2-category” in the general sense of referring to whatever model one may prefer, but usually one in which composition is weak; a bicategory is an adequate definition. One should beware, however, that in the literature it is common for “2-category” to refer only to strict 2-categories.
A 2-category in which all 1-morphisms and 2-morphisms are invertible is a 2-groupoid.
The archetypical 2-category is Cat, the 2-category whose
objects are categories;
morphisms are functors;
2-morphisms are natural transformation;
horizontal composition of 2-morphisms is the Godement product.
This happens to be a strict 2-category.
More generally, for any enriching category (such as a Benabou cosmos), there is a 2-category whose
On the other hand, for any such we also have a bicategory -Prof whose
If is a category with pullbacks, then there is a bicategory Span whose
Every monoidal category may be thought of as a bicategory (its delooping). This has
a single object ;
morphisms are the objects of : ;
2-morphisms are the morphisms of : ;
horizontal composition in is the tensor product in and vertical composition in is composition in .
Conversely, every 2-category with a single object comes from a monoidal category this way, so the concepts are effectively equivalent. (Precisely: the 2-category of pointed 2-categories with a single object is equivalent to that of monoidal categories). For more on this relation see delooping hypothesis, k-tuply monoidal n-category, and periodic table.
Every 2-groupoid is a 2-category. For instance
for any abelian group, the double delooping is the strict 2-category with a single object, a single 1-morphisms, set of 2-moprhisms being and both horizontal composition as well as vertical composition being the product in .
for any 2-group, its single delooping is a 2-groupoid with a single object.
Every topological space has a path 2-groupoid.
Every (∞,2)-category has a homotopy 2-category, obtained by dividing out all 3-morphisms and higher.
An ordinary category has a nerve which is a simplicial set. For 2-categories one may consider their double nerve which is a bisimplicial set.
There is also a 2-nerve. (LackPaoli)
(…)
There is a model category structure on 2-categories – sometimes known as the folk model structure – that models the (2,1)-category underlying 2Cat (Lack).
For strict 2-categories this is the restriction of the corresponding folk model structure on strict omega-categories.
The weak equivalences are the 2-functors that are equivalences of 2-categories.
The acyclic fibrations are the k-surjective functors for all .
Theorem A strict 2-category is cofibrant precisely if the underlying 1-category is a free category.
This is theorem 4.8 in (LackStrict). This is a special case of the more general statement that free strict -categories are given by computads.
Example (free resolution of a 1-category). Let be an ordinary category (a 1-category) regarded as a strict 2-category. Then the cofibrant resolution is the strict 2-category given as follows:
the objects of are those of ;
the morphisms of are finite sequences of composable morphisms of , and composition is concatenation of such sequences
(hence is the free category on the quiver underlying );
the 2-morphisms of are generated from 2-morphisms of the form
and their formal inverses
for all composable with composite (in !) ;
subject to the relation that for all composable triples the following equation of 2-morphisms holds
Observation Let be any strict 2-catgeory. Then a pseudofunctor is the same as a strict 2-functor .
constructions
extra properties
types of morphisms
specific versions
limit notions
model structures
2-category
Early texts
The terminology hypercategory was introduced in
Exposition and review:
Max Kelly and Ross Street, Review of the elements of 2-categories, Sydney Category Seminar 1972/1973, in G. M. Kelly (ed.) Lecture Notes in Mathematics 420, Springer 1974 (doi:10.1007/BFb0063101)
Ross Street, Encyclopedia article on 2-categories and bicategories (pdf)
Tom Leinster, Basic bicategories (arXiv:9810017)
Steve Lack, A 2-categories companion, In: Baez J., May J. (eds.) Towards Higher Categories. The IMA Volumes in Mathematics and its Applications, vol 152. Springer 2010 (arXiv:math.CT/0702535, doi:10.1007/978-1-4419-1524-5_4)
John Power, 2-Categories, BRICS Notes Series 1998 (pdf)
Comprehensive textbook accounts:
Ofer Gabber, Lorenzo Ramero, Chapter 2 of: Foundations for almost ring theory (arXiv:math/0409584)
Niles Johnson, Donald Yau, 2-Dimensional Categories, Oxford University Press 2021 (arXiv:2002.06055, doi:10.1093/oso/9780198871378.001.0001)
Relation between bicategories and Tamsamani weak 2-categories:
Steve Lack, Simona Paoli, 2-nerves for bicategories (arXiv)
Simona Paoli, From Tamsamani weak 2-categories to bicategories (arXiv)
There is a model category structure on 2-categories – the canonical model structure – that models the (2,1)-category underlying 2Cat:
Steve Lack, A Quillen Model Structure for 2-Categories, K-Theory 26: 171–205, 2002. (website)
Steve Lack, A Quillen Model Structure for Biategories, K-Theory 33: 185-197, 2004. (website)
Discussion of weak 2-categories in the style of A-infinity categories is (using dendroidal sets to model the higher operads) in
Andor Lucacs, Dendroidal weak 2-categories (arXiv:1304.4278)
Jonathan Chiche, La théorie de l’homotopie des 2-catégories, thesis, arXiv.
Last revised on March 5, 2023 at 13:59:32. See the history of this page for a list of all contributions to it.