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Results tagged with homotopy-theory
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user 22131
Homotopy theory is an important sub-field of algebraic topology. It is mainly concerned with the properties and structures of spaces which are invariant under homotopy. Chief among these are the homotopy groups of spaces, specifically those of spheres. Homotopy theory includes a broad set of ideas and techniques, such as cohomology theories, spectra and stable homotopy theory, model categories, spectral sequences, and classifying spaces.
6
votes
Accepted
When does a cofibrantly generated model category have this factorization property?
I've encountered that condition a few time. Here is what I know about it:
If that property is satisfied for a model category $M$ then the full subcategory $C$ of finite $I$-cell complex is itself a Br …
- 42.4k
47
votes
What is the mistake in the proof of the Homotopy hypothesis by Kapranov and Voevodsky?
It's been more than a year and a half since I asked this question and I had a lot of thought about it so I decided I will post my own answer.
First I entirely agree with Yonatan that the main problem …
- 12.9k
9
votes
Conservative cocompletion of categories of geometric shapes for homotopy theory
I don't really see a way to give a single answer here, these are 7 different questions (well actually a lot more than 7 if we count all the different flavours of cubes, and of the other ones, probably …
- 42.4k
9
votes
A possible alternative model for $\infty$-groupoids
It is known that the category of finite non-empty set is a test category, in particular there exists a model structure on the category of presheaf on $Fin_+$ whose cofibrations are the monomorphisms a …
- 42.4k
22
votes
Accepted
Useful ideas in category theory which violate the principle of equivalence
I would think about this question in this way: If you have a construction that violates the equivalence principle then either (A) it is a strictified or simplified version of something that is compati …
- 12.9k
13
votes
sSet-enriched categories, quasi-categories and the model-independent theory
This has not been done, and there are good reasons for it: While $sSet$-enriched categories are indeed very good to easily get examples of $\infty$-categories, they are very bad at understanding what …
- 42.4k
8
votes
Accepted
Is the suspension of a finite fibration again finite?
Assuming you work with unpointed spaces (but the example can easily be adapted to the pointed case) the map $1 \to 2$ gives a counterexample : its fiber are $1$ and $\varnothing$ so they are both fini …
- 42.4k
6
votes
Accepted
Localisation of categories, but instead of "isomorphisms" I want "morphisms with right inver...
I think there is a relatively good reason why such a thing shouldn't exists.
In general when you freely add right inverse or inverse, the general arrows of the resulting category will be zig-zag in th …
- 42.4k
12
votes
Accepted
Simple example of nontrivial simplicial localization
For any $1$-category $C$ the localization $C[C^{-1}]$ at all arrows is an $\infty$-groupoid homotopy equivalent to the nerve of $C$, so it can be any $\infty$-groupoid.
For example take $C$ to be the …
- 30.3k
2
votes
Accepted
Does the monoidal structure on semisimplicial sets preserve fibrant objects?
It is not the case: the terminal semi-simplicial set $1$ is obviously fibrant but as I will show below the geometric product $1 \otimes 1$ is not fibrant.
1) What does $1 \otimes 1$ look like ?
So, $1 …
- 42.4k
3
votes
Accepted
Universal model category as a $\text{sSet}$-enriched co-completion
Given $C$ a small category (eventually, a small simplicial category) I denote by $UC$ the projective model structure on the category of simplicial presheaves on $C$ as in the paper. Using the kind of …
- 30.3k
14
votes
Why is Kan's $Ex^\infty$ functor useful?
Another thing for which Kan Ex$^{\infty}$ functor is useful is actually in the construction of the Kan-Quillen model structure. It morally gives a purely algebraic version of the simplicial approximat …
- 42.4k
13
votes
Correspondence between classes of model categories and classes of $\infty$-categories
Regarding (1) :
A) Every model category has an associated $\infty$-category, obtained for example by taking the Dwyer-Kan localization at the class of all equivalence, (But there are other more expli …
- 42.4k
1
vote
Accepted
Does a homotopy sheaf functor commute with group completion
I'm still confused by the question when $n>0$: $\pi_n^{\tau} X $ are already groups, so applying the group completion functor do not do anything to them, and, even in the catagory of spaces, I have ne …
- 42.4k
23
votes
What is the intuition for higher homotopy groups not vanishing?
So far the discusion mostly focused on a geometric explanation, I'd like to mention the algebraic one as well:
One way to formulate it involves the delooping machinery: up to delooping, $\mathbb{S}^n …
- 42.4k