Homotopy hypothesis

In category theory, a branch of mathematics, Grothendieck's homotopy hypothesis states, homotopy theory speaking, that the ∞-groupoids are spaces.

One version of the hypothesis was claimed to be proved in the 1991 paper by Kapranov and Voevodsky. Their proof turned out to be flawed and their result in the form interpreted by Carlos Simpson is now known as the Simpson conjecture.

In higher category theory, one considers a space-valued presheaf instead of a set-valued presheaf in ordinary category theory. In view of homotopy hypothesis, a space here can be taken to an ∞-groupoid.

Formulations

A precise formulation of the hypothesis very strongly depends on the definition of an ∞-groupoid. One definition is that, mimicking the ordinary category case, an ∞-groupoid is an ∞-category in which each morphism is invertible or equivalently its homotopy category is a groupoid.

Now, if an ∞-category is defined as a simplicial set satisfying the weak Kan condition, as done commonly today, then ∞-groupoids amounts exactly to Kan complexes (= simplicial sets with the Kan condition) by the following argument. If $X$ is a Kan complex (viewed as an ∞-category) and $f$ a morphism in it, consider $\sigma :\Lambda _{0}^{2}\to X$ from the horn such that $\sigma (0\to 1)=f,\,\sigma (0\to 2)=\operatorname {id}$. By the Kan condition, $\sigma$ extends to ${\overline {\sigma }}:\Delta ^{2}\to X$ and the image $g={\overline {\sigma }}(1\to 2)$ is a left inverse of $f$. Similarly, $f$ has a right inverse and so is invertible. The converse, that an ∞-groupoid is a Kan complex, is less trivial and is due to Joyal.

Because of the above fact, it is common to define ∞-groupoids simply as Kan complexes. Now, a theorem of Milnor says that Kan complexes completely determine the homotopy theory of (reasonable) topological spaces. So, this essentially proves the hypothesis. In particular, if ∞-groupoids are defined as Kan complexes (bypassing Joyal’s result), then the hypothesis is almost trivial.

However, if an ∞-groupoid is defined in different ways, then the hypothesis is usually still open. In particular, the hypothesis with Grothendieck's original definition of an ∞-groupoid is still open.

n-version

There is also a version of homotopy hypothesis for n-groupoids, which roughly says

Homotopy hypothesis—An n-groupoid is exactly the same as a homotopy n-type.

The statement requires several clarifications:

• An n-groupoid is typically defined as an n-category where each morphism is invertible. So, in particular, the meaning depends on the meaning of an n-category (e.g., usually some weak version of an n-category),
• "the same as" usually means some equivalence (not necessarily strong), and the definition of an equivalence typically uses some higher notions like an ∞-category,
• A homotopy n-type means a reasonable topological space with vanishing i-th homotopy groups, i > n at each base point.

This version is still open.

See also

• Pursuing Stacks
• N-group (category theory)
• Quasi-category
• Algebraic homotopy

References

• John Baez, The Homotopy Hypothesis
• Baez, John C. (1997). "An introduction to n-categories". Category Theory and Computer Science. Lecture Notes in Computer Science. Vol. 1290. pp. 1–33. arXiv:q-alg/9705009. doi:10.1007/BFb0026978. ISBN 978-3-540-63455-3.
• Grothendieck, Alexander (2021). "Pursuing Stacks". arXiv:2111.01000 [math.CT].
• Gurski, Nick; Johnson, Niles; Osorno, Angélica M. (2019). "The 2-dimensional stable homotopy hypothesis". Journal of Pure and Applied Algebra. 223 (10): 4348–4383. arXiv:1712.07218. doi:10.1016/j.jpaa.2019.01.012.
• Joyal, A. (2002). "Quasi-categories and Kan complexes". Journal of Pure and Applied Algebra. 175 (1–3): 207–222. doi:10.1016/S0022-4049(02)00135-4.
• Lurie, Jacob (2009). Higher Topos Theory (AM-170). Princeton University Press. ISBN 9780691140490. JSTOR j.ctt7s47v.
• Land, Markus (2021). "Joyal's Theorem, Applications, and Dwyer–Kan Localizations". Introduction to Infinity-Categories. Compact Textbooks in Mathematics. pp. 97–161. doi:10.1007/978-3-030-61524-6_2. ISBN 978-3-030-61523-9. Zbl 1471.18001.
• Maltsiniotis, Georges (2010). "Grothendieck $\infty$-groupoids, and still another definition of $\infty$-categories, §2.8. Grothendieck's conjecture (precise form)". arXiv:1009.2331 [math.CT].
• Nikolaus, Thomas (2011). "Algebraic models for higher categories". Indagationes Mathematicae. 21 (1–2): 52–75. arXiv:1003.1342. doi:10.1016/j.indag.2010.12.004.
• Riehl, Emily (2023). "Could ∞-Category Theory be Taught to Undergraduates?". Notices of the American Mathematical Society. 70 (5): 1. doi:10.1090/noti2692.
• Tamsamani, Zouhair (1999). "Sur des notions de n-categorie et n-groupoide non strictes via des ensembles multi-simpliciaux (On the notions of a nonstrict n-category and n-groupoid via multisimplicial sets)". K-Theory. 16: 51–99. arXiv:alg-geom/9512006. doi:10.1023/A:1007747915317.
• John C. Baez and Michael Shulman, Lectures on 𝑛-categories and cohomology, Towards higher categories, IMA Vol. Math. Appl., vol.152, Springer, New York, 2010, pp. 1–68. MR2664619

Further reading

• Ayala, David; Francis, John; Rozenblyum, Nick (2018). "A stratified homotopy hypothesis". Journal of the European Mathematical Society. 21 (4): 1071–1178. arXiv:1502.01713. doi:10.4171/JEMS/856.

External links

• homotopy hypothesis at the nLab
• "What is the mistake in the proof of the Homotopy hypothesis by Kapranov and Voevodsky?". MathOverflow.
• Jacob Lurie's Home Page
• Henry, Simon (May 26, 2022). Grothendieck's homotopy hypothesis (PDF). Grothendieck, a Multifarious Giant: Mathematics, Logic and Philosoph.
• "Current status of Grothendieck's homotopy hypothesis and Whitehead's algebraic homotopy programme". MathOverflow.