In mathematics, higher category theory is the part of category theory at a higher order, which means that some equalities are replaced by explicit arrows in order to be able to explicitly study the structure behind those equalities. Higher category theory is often applied in algebraic topology (especially in homotopy theory), where one studies algebraic invariants of spaces, such as their fundamental weak ∞groupoid.
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Strict higher categories
An ordinary category has objects and morphisms, which are called 1morphisms in the context of higher category theory. A 2category generalizes this by also including 2morphisms between the 1morphisms. Continuing this up to nmorphisms between (n − 1)morphisms gives an ncategory.
Just as the category known as Cat, which is the category of small categories and functors is actually a 2category with natural transformations as its 2morphisms, the category nCat of (small) ncategories is actually an (n + 1)category.
An ncategory is defined by induction on n by:
So a 1category is just a (locally small) category.
The monoidal structure of Set is the one given by the cartesian product as tensor and a singleton as unit. In fact any category with finite products can be given a monoidal structure. The recursive construction of nCat works fine because if a category C has finite products, the category of Cenriched categories has finite products too.
While this concept is too strict for some purposes in for example, homotopy theory, where "weak" structures arise in the form of higher categories,^{[1]} strict cubical higher homotopy groupoids have also arisen as giving a new foundation for algebraic topology on the border between homology and homotopy theory; see the article Nonabelian algebraic topology, referenced in the book below.
Weak higher categories
In weak ncategories, the associativity and identity conditions are no longer strict (that is, they are not given by equalities), but rather are satisfied up to an isomorphism of the next level. An example in topology is the composition of paths, where the identity and association conditions hold only up to reparameterization, and hence up to homotopy, which is the 2isomorphism for this 2category. These nisomorphisms must well behave between homsets and expressing this is the difficulty in the definition of weak ncategories. Weak 2categories, also called bicategories, were the first to be defined explicitly. A particularity of these is that a bicategory with one object is exactly a monoidal category, so that bicategories can be said to be "monoidal categories with many objects." Weak 3categories, also called tricategories, and higherlevel generalizations are increasingly harder to define explicitly. Several definitions have been given, and telling when they are equivalent, and in what sense, has become a new object of study in category theory.
Quasicategories
Weak Kan complexes, or quasicategories, are simplicial sets satisfying a weak version of the Kan condition. André Joyal showed that they are a good foundation for higher category theory. Recently, in 2009, the theory has been systematized further by Jacob Lurie who simply calls them infinity categories, though the latter term is also a generic term for all models of (infinity, k) categories for any k.
Simplicially enriched categories
Simplicially enriched categories, or simplicial categories, are categories enriched over simplicial sets. However, when we look at them as a model for (infinity, 1)categories, then many categorical notions (e.g., limits) do not agree with the corresponding notions in the sense of enriched categories. The same for other enriched models like topologically enriched categories.
Topologically enriched categories
Topologically enriched categories (sometimes simply called topological categories) are categories enriched over some convenient category of topological spaces, e.g. the category of compactly generated Hausdorff spaces.
Segal categories
These are models of higher categories introduced by Hirschowitz and Simpson in 1998,^{[2]} partly inspired by results of Graeme Segal in 1974.
See also
Notes
 ^ Baez & Dolan 1998, p. 6
 ^ Hirschowitz, André; Simpson, Carlos (2001). "Descente pour les nchamps (Descent for nstacks)". arXiv:math/9807049.
References
 Baez, John C.; Dolan, James (1998). "Categorification". arXiv:math/9802029.
 Leinster, Tom (2004). Higher Operads, Higher Categories. Cambridge University Press. arXiv:math.CT/0305049. ISBN 0521532159.
 Simpson, Carlos (2010). "Homotopy theory of higher categories". arXiv:1001.4071 [math.CT]. Draft of a book. Alternative PDF with hyperlinks)
 Lurie, Jacob (2009). Higher Topos Theory. Princeton University Press. arXiv:math.CT/0608040. ISBN 9780691140483. As PDF.
 nLab, the collective and open wiki notebook project on higher category theory and applications in physics, mathematics and philosophy
 Joyal's Catlab, a wiki dedicated to polished expositions of categorical and higher categorical mathematics with proofs
 Brown, Ronald; Higgins, Philip J.; Sivera, Rafael (2011). Nonabelian algebraic topology: filtered spaces, crossed complexes, cubical homotopy groupoids. Tracts in Mathematics. Vol. 15. European Mathematical Society. ISBN 9783037190838.
External links
 Baez, John (24 February 1996). "Week 73: Tale of nCategories".
 The nCategory Cafe — a group blog devoted to higher category theory.
 Leinster, Tom (8 March 2010). "A Perspective on Higher Category Theory".