nLab infinity-groupoid

Idea

The notion of an $\infty$-groupoid is a generalization of that of a group and groupoids to higher category theory.

An $\infty$-groupoid – equivalently an (∞,0)-category – is an ∞-category in which, for all natural numbers $k$, all k-morphisms are equivalences.

The collection of all $\infty$-groupoids forms the (∞,1)-category ∞Grpd.

Special cases of $\infty$-groupoids include groupoids, 2-groupoids, 3-groupoids, n-groupoids, deloopings of groups, 2-groups, and ∞-groups.

Properties

Presentations

There are many ways to model the (∞,1)-category ∞Grpd of all $\infty$-groupoids, or at least obtain its homotopy category.

A simple and very useful incarnation of $\infty$-groupoids is available using a geometric definition of higher categories in the form of simplicial sets that are Kan complexes. The $k$-cells of the underlying simplicial set are the k-morphisms of the $\infty$-groupoid, and the Kan horn-filler conditions encode the fact that adjacent $k$-morphisms have a (non-unique) composite $k$-morphism and that every $k$-morphism is invertible with respect to this composition. See Kan complex for a detailed discussion of how these incarnate $\infty$-groupoids.

The (∞,1)-category of all $\infty$-groupoids is modelled along these lines by the Quillen model structure on simplicial sets, whose fibrant-cofibrant objects are precisely the Kan complexes:

$$\infty Grpd \simeq (sSet_{Quillen})^\circ.$$

One may turn this geometric definition into an algebraic definition of ∞-groupoids by choosing horn-fillers . The resulting notion is that of an algebraic Kan complex that has been shown by Thomas Nikolaus to yield an equivalent (∞,1)-category of $\infty$-groupoids.

There are various model categories which are Quillen equivalent to $sSet_{Quillen}$. For instance, the standard model structure on topological spaces, a model structure on marked simplicial sets, and many more. All these present ∞Grpd.

Moreover, the corresponding homotopy category of an (∞,1)-category $Ho(\infty Grpd)$, a category whose objects are the homotopy types of the $\infty$-groupoids, is given by the homotopy category of the category of presheaves over any test category. See there for more details.

Every other algebraic definition of an omega-category is supposed to yield an equivalent notion of an $\infty$-groupoid when restricted to $\omega$-categories whose k-morphisms are all invertible. This is the statement of the homotopy hypothesis, which is a theorem for Kan complexes and algebraic Kan complexes and regarded as a consistency condition for other definitions.

Notably, in Pursuing Stacks and the earlier letter to Larry Breen, Alexander Grothendieck initiated the study of $\infty$-groupoids and the homotopy hypothesis with his original definition of Grothendieck weak infinity-groupoids, which has recently attracted renewed attention.

Strict $\infty$-groupoids

One may also consider entirely strict $\infty$-groupoids, usually called $\omega$-groupoids or strict ∞-groupoids. These are equivalent to crossed complexes of groups and groupoids.

Relation to $\infty$-groups

Pointed0-connected $\infty$-groupoids are the deloopings of ∞-groups. (See looping and delooping.)

These are presented by simplicial groups. Notably, abelian simplicial groups are a model for abelian $\infty$-groupoids (more precisely, H$\mathbb{Z}$-modules). Under the Dold-Kan correspondence, these are equivalent to non-negatively graded chain complexes, which are therefore also a model for abelian $\infty$-groupoids. In this way, much of homological algebra is really the study of special (structured) $\infty$-groupoids.

Related concepts

• discrete ∞-groupoid, ∞-stack, cohesive ∞-groupoid

• homotopy theory, (∞,1)-category

• model structure for ∞-groupoids

• ∞Grpd

• simplicial infinity-groupoid

• homotopy hypothesis, test category, modelizer

homotopy level	n-truncation	homotopy theory	higher category theory	higher topos theory	homotopy type theory
h-level 0	(-2)-truncated	contractible space	(-2)-groupoid		true/​unit type/​contractible type
h-level 1	(-1)-truncated	contractible-if-inhabited	(-1)-groupoid/​truth value	(0,1)-sheaf/​ideal	mere proposition/​h-proposition
h-level 2	0-truncated	homotopy 0-type	0-groupoid/​set	sheaf	h-set
h-level 3	1-truncated	homotopy 1-type	1-groupoid/​groupoid	(2,1)-sheaf/​stack	h-groupoid
h-level 4	2-truncated	homotopy 2-type	2-groupoid	(3,1)-sheaf/​2-stack	h-2-groupoid
h-level 5	3-truncated	homotopy 3-type	3-groupoid	(4,1)-sheaf/​3-stack	h-3-groupoid
h-level $n+2$	$n$-truncated	homotopy n-type	n-groupoid	(n+1,1)-sheaf/​n-stack	h-$n$-groupoid
h-level $\infty$	untruncated	homotopy type	∞-groupoid	(∞,1)-sheaf/​∞-stack	h-$\infty$-groupoid

Terminology

The term “∞-groupoid” is sometimes considered to be too unwieldy, and some alternatives have been suggested or used, but none has gained wide acceptance.

Historically, the word “space” is often (ab)used to mean “∞-groupoid”, due to the traditional presentation of $\infty Gpd$ by the model structure on topological spaces. Some have condemned this usage, but others argue that a “homotopy space” is a valid notion of a “space”.

The term “space” is also often used to refer to simplicial sets. In particular, Bousfield and Kan in their book “Homotopy Limits, Completions and Localizations” write:

“These notes are written simplicially, i.e. whenever we say space we mean simplicial set.

More recently, Jacob Lurie‘s work continues this usage.

The term “homotopy type” is also quite close in meaning to “∞-groupoid”. Historically, it differed in that morphisms of homotopy types were mere homotopy classes of maps of ∞-groupoids, but, more recently (especially with the advent of homotopy type theory), some have used “homotopy type” synonymously with “$\infty$-groupoid”.

More radically, Dustin Clausen and Peter Scholze use the term “anima” (plural: anima) as a synonym for “$\infty$-groupoid”, in particular in relation to condensed mathematics. Reference: nCafe. In Purity for flat cohomology by Kęstutis Česnavičius and Peter Scholze they write “If the objects of $C$ are called widgets, then we call those of $Ani(C)$ animated widgets, except that we abbreviate $Ani(Set)$ to $Ani$ and the term ‘animated set’ to anima (plural: anima).”

References

• Jacob Lurie, Section 1.1.2 in: Higher Topos Theory, Annals of Mathematics Studies 170, Princeton University Press 2009 (pup:8957, pdf)

Formulations in/from homotopy type theory:

• Benno van den Berg, Richard Garner, Types are weak omega-groupoids, Proceedings of the London Mathematical Society 102 2 (2011) 370-394 [arXiv:0812.0298, doi:10.1112/plms/pdq026]

• Thorsten Altenkirch, Ondrej Rypácek, A Syntactical Approach to Weak $\omega$-Groupoids, Computer Science Logic (CSL’12) 16 (2012) 16-30 [pdf, doi:10.4230/LIPIcs.CSL.2012.16]

See also at category object in an (infinity,1)-category for more along these lines.

category: ∞-groupoid