fundamental infinity-groupoid in nLab

Contents

Contents

Idea

The fundamental ∞\infty-groupoid Π ∞(X)\Pi_\infty(X) of a topological space XX is the ∞-groupoid whose k-morphisms are the kk-dimensional paths in XX. This is the higher refinement of the fundamental groupoid Π 1(X)\Pi_1(X).

It is also sometimes called the ∞\infty-Poincaré groupoid of the space, in analogy to the term Poincaré groupoid for the fundamental groupoid.

Definition

General version

The following definition is appropriate if we take a Kan complex as the definition of ∞\infty-groupoid.

For other models of ∞Grpd there are correspondingly other constructions:

• The definition of Trimble n-category has the concept of fundamental nn-groupoid built right into it.

Strict versions

One can consider strict ∞-groupoid versions of the fundamental ∞\infty-groupoid. These lose information about the homotopy type of the space, though, but are more tractable and may give in some applications all the information that one is interested in.

The study of strict fundamental ∞\infty-groupoids have been pursued by Ronnie Brown and his school.

There is a strict homotopy 2-groupoid for a Hausdorff space defined spring , and a weak homotopy 2-groupoid for a general space (by the same authors). They later introduced a homotopy double groupoid. There is no nn-dimensional version of these ideas on offer.

A strict cubical omega-groupoid ρX *\rho X_* for a filtered space X *X_* was defined by Brown and Higgins in 1981. Form the filtered cubical complex RX *R X_* which in dimension nn consists of filtered maps I * n→X *I^n_* \to X_* and take filter homotopy classes of these relative to the vertices. The proof that the compositions in RX *RX_* are inherited by ρX *\rho X_* is one of the key points of the development.

It turns out that ρX *\rho X_* is equivalent in a clear sense to the crossed complex ΠX *\Pi X_* defined using relative homotopy groups by Blakers in 1948 (with other terminology) and that the homotopy types modelled by crossed complexes, or by the corresponding globular or cubical gadget, are restricted, essentially to the linear homotopy types, with no quadratic information. Nonetheless, it is well known in mathematics that linear approximations can be useful.

Loday’s paper of 1982 on Spaces with finitely many homotopy groups introduced the entirely new idea of a cubical resolution of a space. Some details were completed by Richard Steiner. Loday also introduced the fundamental cat-n-group of an nn-cube of spaces. In this way we get a model of a space XX by a multiple groupoid in which the rr-dimensional homotopy of XX occurs in the right place in the model. Also you can calculate something with this model, and it has led to new algebraic constructions, such as a nonabelian tensor product of groups, with homotopical applications.

These strict groupoid models do satisfy the dimension condition.

Properties

• fundamental groupoid

• fundamental ∞\infty-groupoids incarnated as sSet-enriched groupoids aka “Dwyer-Kan simplicial groupoids” are known as Dwyer-Kan loop groupoids.

• fundamental category, fundamental (∞,1)-category

• fundamental ∞-groupoid in a locally ∞-connected (∞,1)-topos / of a locally ∞-connected (∞,1)-topos

• shape via cohesive path ∞-groupoid

References

The fundamental 2-groupoid:

as a strict 2-groupoid:

• Keith A. Hardie, Klaus H. Kamps, Rudger Kieboom, A homotopy 2-groupoid of a Hausdorff space. Papers in honour of Bernhard Banaschewski (Cape Town, 1996). Appl. Categ. Structures 8 (2000) 209-234 [doi:10.1023/A:1008758412196]

as a weak 2-groupoid:

• Keith A. Hardie, Klaus H. Kamps, Rudger Kieboom, A Homotopy Bigroupoid of a Topological Space, Applied Categorical Structures 9 (2001) 311-327 [doi:10.1023/A:1011270417127]

See also:

• J.-L. Loday, Spaces with finitely many homotopy groups, J. Pure Appl. Alg., 24 (1982) 179–202.

• J. M. Casas, G. Ellis, M. Ladra, T. Pirashvili, Derived functors and the homology of nn-types, J. Algebra 256, 583–598 (2002).

The direct proof of the homotopy hypothesis for the algebraic version of the fundamental ∞\infty-groupoid is in

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

Strict versions of fundamental ∞\infty-groupoids are discussed in

• Ronnie Brown et al. Nonabelian Algebraic Topology

See also

• Ronnie Brown and Philip Higgins, Colimit theorems for relative homotopy groups, J. Pure Appl. Algebra 22 (1981) 11-41.

• Ronnie Brown, A new higher homotopy groupoid: the fundamental globular ω\omega-groupoid of a filtered space, Homotopy, Homology and Applications, 10 (2008), No. 1, pp.327-343.

• Richard Steiner, Resolutions of spaces by nn-cubes of fibrations, J. London Math. Soc.(2), 34, 169-176, 1986.