homotopy hypothesis in nLab

Contents

1. Idea

The homotopy hypothesis is the assertion that

or rather the stronger statement that

and moreover

• this equivalence is induced by the fundamental ∞-groupoid construction.

What this actually means in detail depends on which definition of ∞Grpd is being used and to which precise incarnation of Top it is being compared to.

There are some definitions of ∞-groupoids for which the homotopy hypothesis is a proven theorem . Depending on where in the spectrum between geometric definitions of higher categories and algebraic definitions of higher categories a given definition of ∞-groupoids is located, the statement may be more or less obvious.

For instance there is some justification for defining an ∞-groupoid to be equivalently a topological space (considered modulo weak homotopy equivalence). For this definition the homotopy hypothesis is of course a tautology.

A definition of ∞-groupoid that is still very geometrical but much more combinatorial is that given by Kan complexes. For these the homotopy hypothesis has a (non-trivial but fairly tractable) proof. The equivalence between Kan complexes and CW-complexes obtained this way is at the heart of all traditional homotopy theory.

A genuine algebraic definition of ∞-groupoids for which the homotopy theory has a (non-trivial but tractable) proof is given by algebraic Kan complexes.

However for other algebraic definitions of ∞-groupoids not much indication for how to prove the homotopy hypothesis is known. The definition of Trimble ∞-category stands out as an algebraic definition that has the notion of fundamental ∞-groupoid built right into it, but also here it seems unclear at the moment how to make progress with proving the homotopy hypothesis.

In fact, generally the homotopy hypothesis is regarded as a consistency condition for definitions in higher category theory:

• Any definition of (n,r)-category should be such that there is a fundamental n-groupoid-construction with values in the corresponding (n,0)-categories which makes these equivalent to homotopy n-types.

One way to justify this condition is by recourse to the proven cases of the homotopy hypothesis: experience shows that the collections of all three models – topological spaces, Kan complexes, algebraic Kan complexes – provide a model for ∞Grpd that supports the general abstract higher category theory – specifically (∞,1)-category theory – that one expects in analogy to how Set supports ordinary category theory. Any other definition of ∞-groupoids is hoped/required to reproduce this, and hence is hoped/required to satisfy the homotopy hypothesis.

Apart from having different models of ∞-groupoids that lend themselves more or less to a comparison with topological spaces, there is also the issue as to how to conceive of the notion of equivalence between ∞-groupoids.

The usual, unstated, implication is that the notion of equivalence of n-groupoids used to model homotopy n-types is the appropriate n-category-theoretic notion of equivalence. It is in this way that, for instance, it is known that 1-groupoids model homotopy 1-types (see below).

The reason this is important to specify is that there are other notions of equivalence on categorical structures which model homotopy types in other ways. For example, if we declare a functor between categories to be a weak equivalence iff its nerve is a weak equivalence of simplicial sets, then all homotopy types can be modeled by 1-categories in this way; see the Thomason model structure for 1-categories.

Finally, in analogy to the homotopy hypothesis, there are also attempts to relate general (∞,n)-categories (not necessarily groupoidal) to directed topological spaces by a fundamental (∞,n)-category-construction. There have been claims that a directed homotopy hypothesis can be proven, but at the moment there does not seem to be a published statement.

2. Abstract statement

The following general abstract statement of the homotopy hypothesis is often useful to make explicit.

This statement can be formulated, holds true and is proven below at least for the standard definitions of these two (∞,1)-categories (see section on Kan complexes, section on algebraic Kan complexes).

3. Realizations

We discuss various different definitions of n-groupoids and ∞-groupoids and the formulation and proof of the homotopy hypothesis for them, to the extent that it is available.

For groupoids

See homotopy hypothesis for 1-types for more.

For 2-groupoids

(…) 2-groupoids model all homotopy 2-types (…)

strict 2-groupoids suffice (…) (but note that strict 2-functors are not sufficient to model all maps between 2-types)

For Gray-groupoids

(…) Gray-groupoids model all homotopy 3-types (…)

It is known that not all homotopy 3-types can be modeled by strict 3-groupoids, but that Gray-categories (semi-strict 3-categories) suffice; the obstruction is the Whitehead product which arises from a nontrivial interchanger.

For Kan complexes

We write sSetQuillen for sSet equipped with the classical model structure on simplicial sets. The cofibrant-fibrant objects in sSetQuillen are precisely the Kan complexes.

Also we write TopQuillen for Top equipped with the classical model structure on topological spaces.

(Quillen 67), see e.g. (Goerss-Jardine 96, section I.11, Joyal-Tierney 05, chapter I)

For algebraic Kan complexes

An algebraic Kan complex is an algebraic definition of higher groupoids obtained by taking the ordinary definition of Kan complex and equipping these with choices of horn-fillers. These choices encode specified composition operations, specified associators for these, specified pentagonators and so on.

Algebraic Kan complexes constitute a genuine algebraic model in that they are precisely the algebras over a monad on sSet.

But there is also a direct Quillen equivalence:

This is (Nikolaus, prop. 3.4).

This is (Nikolaus, corollary 3.5)

This is Nikolaus, corollary 3.6

For cubical sets

Also cubical sets may serve as a model for homotopy theory.

There is an evident simplicial set-valued functor

from the cube category to sSet, which sends the cubical n-cube to the simplicial n-cube

Similarly there is a canonical Top-valued functor

The corresponding nerve and realization adjunction

is the cubical analogue of the simplicial nerve and realization discussed above.

This is Jardine, sections 3.

This is Jardine, theorem 29, corollary 30.

For catn groups and n+1-fold groupoids

Loday’s notion of a cat-n-group corresponds to the connected version of an n+1-fold groupoid. We will restrict our discussion to that connected case.

This is proven in (Loday). (There are some glitches in his proof and these were fixed by various authors (Steiner, Gilbert, ..) and then detailed proofs were given by Bullejos, Cegarra, Duskin and separately, using the equivalent formulation of crossed n-cubes, by Porter. Detailed references and some more commentary is at cat-n-group.)

For Segal-groupoids

For strict ω-categories with weak inverses

While strict omega-groupoids in the sense of strict omega-categories with strict inverses are far from modelling all homotopy types, strict ω-categories with all weak inverses come closer. In (Kapranov-Voevodsky) it was argued that these are in fact sufficient, but a mistake in the argument is claimed in (Simpson, cor. 5.2) (see also here).

The issue however is somewhat subtle, as very much highlighted by Voevodsky here. For more on this see at Simpson's conjecture.

4. Generalizations

For stratified spaces

For a categorified version which finds an equivalence for (∞,1)-categories, see stratified homotopy hypothesis (Ayala-Francis-Rozenblyum).

For spectra

For a stabilized version which finds an equivalence between stable homotopy types (i.e. spectra) and symmetric monoidal ∞-groupoids, see stable homotopy hypothesis (Johnson-Osorno, Gurski-Johnson-Osorno)

5. References and a bit of history.

The equivalence between the classical homotopy theory of topological spaces and the homotopy theory of Kan complexes is due to

• Daniel Quillen, Homotopical Algebra, LNM 43, Springer, (1967)

Later in

• Alexander Grothendieck, Pursuing Stacks, 1983 (including and especially the ‘letter to Daniel Quillen’ given on the first few pages),

it was argued that there were algebraic definitions of higher groupoids that could be put forward, so that the resulting objects ought to have a homotopy theory equivalent to the classical homotopy theory of topological spaces, and, moreover, would provide useful tools in the interpretation of non-abelian cohomology classes. This extended ideas that Grothendieck had explored in letters to Larry Breen in the mid 1970s in which he had given a sketch of a theory of n-stacks and their relation with the homotopy n-type of a space or more generally a topos.

At this stage, (in the 1970s and early 1980s) more geometric or combinatorial definitions of infinity categories were not yet available, or, perhaps more accurately, had been discovered, but were not recognised as having such an infinity categoric interpretation; see Tim Porter‘s Letter to Grothendieck (16 June 1983) and the discussion here in New Spaces for Mathematics and Physics. These models included Kan complexes which now are interpreted as being one model for infinity groupoids, and weak Kan complexes, as put forward by Boardman and Vogt, which give one of the main models for (weak) (∞,1)-categories. From this perspective, Quillen’s result can be seen as being a precursor of one form of the Homotopy Hypothesis.

The name “homotopy hypothesis” for this statement is due to

• John Baez, The Homotopy Hypothesis (web, pdf)

Technical reviews of Quillen’s proof of the homotopy hypothesis includes

The homotopy hypothesis for strict ω-categories with weak inverses is discussed in

• Mikhail Kapranov, Vladimir Voevodsky, ∞-groupoids and homotopy types, Cahiers de Topologie et Géométrie Différentielle Catégoriques, 32 no. 1 (1991), p. 29-46

but a mistake in the argument is claimed in cor 5.2 of

• Carlos Simpson, Homotopy types of strict 3-groupoids (arXiv:math/9810059)

The homotopy hypothesis for algebraic Kan complexes is established and discussed in

• Thomas Nikolaus, Algebraic models for higher categories (arXiv)

The homotopy hypothesis for Segal groupoids is formulated in section 6.3.4 of

• Regis Pellissier, Weak enriched categories - Categories enrichies faibles PhD Thesis (2002) (arXiv:math/0308246)

Models of homotopy n-types by Catn-groups are discussed in

• Jean-Louis Loday, Spaces with finitely many non-trivial homotopy groups, J. Pure Appl. Algebra 24 (1982), 179-202.

More literature on models of homotopy types by strict higher groupoids is at

• Ronnie Brown, Computing Homotopy Types Using Crossed N-Cubes of Groups (arXiv)

• Simona Paoli, Internal categorical structures in homotopical algebra, to appear in Towards Higher Categories, eds. John Baez and Peter May (pdf)

The first paper, as its title suggests, has an emphasis on using higher groupoids for computation of homotopical invariants, in fact by applying higher homotopy van Kampen Theorems. These theorems lead to algebraic colimit arguments in algebraic topology, implying results, often nonabelian, not obtainable by other methods. It is also remarkable that the precision of these results requires the use of strict structures, whereas the current emphasis in higher category theory is on non strict structures.

The homotopy theory of cubical sets is discussed in

• Jardine, Model structure on cubical sets (pdf)

• Maltsiniotis, G. La catégorie cubique avec connexions est une catégorie test stricte. Homology, Homotopy Appl. 11, (2) (2009) 309–326.

Cubical methods are also essential in

• R. Brown, P.J. Higgins, R. Sivera, Nonabelian algebraic topology: filtered spaces, crossed complexes, cubical homotopy groupoids, EMS Tracts in Mathematics Vol. 15, 703 pages. (August 2011).

A version for stratified spaces is discussed in

A version for spectra is discussed in

• Niles Johnson and Angelica Osorno?, Modeling stable one-types, tac

• Nick Gurski, Niles Johnson, and Angelica Osorno?, The 2-dimensional stable homotopy hypothesis, arxiv