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Part of higher category theory that for instance in Algebraic Topology enables us to capture finer homotopic distinctions. As in say Eilenberg-Maclane spaces.
2
votes
Coequalizers in stable (infinity,1)-categories
I think you prove this pretty much as one does in an ordinary abelian category.
First note that the coequalizer $f,g: X \rightarrow Y$ is also given by the pushout of $(f, 1)$ and $(g, 1)$, both maps …
2
votes
Accepted
Contractibility of cocartesian liftings
If $p$ is an inner fibration then the answer is yes (HTT.2.4.1.7). Lurie doesn't even define p-cocartesian unless $p$ is an inner fibration, but it's not hard to redefine the notion slightly more gen …
4
votes
Localizations of model categories and $\infty$-categories
Though I don't know of an example off-hand, I don't believe it's the case that every cellular model category presents a presentable $\infty$-category. In practice, we can usually find a combinatorial …
2
votes
Lemma 2.1.1.4 in Lurie's HTT
Consider the diagram
where the top left horizontal arrow is given by $\eta$. Observe that homotopy classes of lifts of the map $X_s \times \Delta^1 \rightarrow S$ are the same as homotopy classes o …
6
votes
What is an example of a quasicategory with an outer 4-horn which has no filler?
Here's a way to cook up lots of examples. Notice that $\Lambda^n_0 = \Delta^0 \star \partial \Delta^{n-1}$ so that a functor $\Lambda^n_0 \to \mathcal{C}$ is the same as a functor $\partial\Delta^{n-1 …
20
votes
Accepted
Stable infinity categories vs dg-categories
Here are a few observations...
I think there exist stable infinity categories that are not the dg-nerve (resp. $A_\infty$-nerve) of a dg-category (resp. $A_\infty$ category). In particular, the cate …
5
votes
On combinatorial and cellular model categories and infinity categories
Maybe I should record these as answers:
(1) If you add "left proper" to the requirement then the answer is that there are lots of examples. A theorem of Dugger says that any left proper, cofibrantly …
5
votes
Accepted
Directed colimits of maps in a combinatorial model category
The answer is 'no' (sorry I didn't realize this before!)
I remembered someone showing me a weird counterexample in model categories a couple weeks ago (elaborated on from this paper by Rosicky: http: …
12
votes
Accepted
On HTT's Lemma 3.3.4.1
The lemma is true as stated. The Cartesian equivalence in question does not imply an equivalence in the Joyal model structure after forgetting the markings; rather, it implies an equivalence after for …
5
votes
Decomposing a (co)limit by decomposing the indexing diagram
The way I always remember this stuff is as follows:
Given a map $J \to \mathsf{Cat}$ form the associated cocartesian fibration $E \to J$.
By assumption, $I$ is the actual colimit (as opposed to the l …
4
votes
Are reflective subcategories of complete infinity categories complete?
Here's a proof which is certainly overkill, but it has the merit of using references so you can read the proofs in detail.
We have $i: \mathcal{C} \subset \mathcal{D}$ a fully faithful subcategory wi …
6
votes
What functors are classified by slices of $\infty$-categories?
I like Maxime's argument better, but here's another.
As you say, the case when $C=\bullet$ is well-known. But we can reduce to that case! The map $D_{/f} \to D$ is pulled back from $\mathsf{Fun}(C, D) …
9
votes
Accepted
How to characterize $E_n$ morphisms from $\mathrm{Mod}(A)$ to $\mathrm{Mod}(B)$?
By Corollary HA.4.8.5.20, the functor from $\mathbb{E}_{n+1}$-algebras to $\mathbb{E}_n$-monoidal categories and colimit-preserving, $\mathbb{E}_n$-monoidal functors is fully faithful. (Notice that th …
7
votes
Accepted
Functorial construction of ("pre"-)spectral sequences? (Or - what is the "higher structure" ...
Let me propose an answer to the question which isn't quite what you ask for. In fact, for the most part I agree with Denis that the correct object really is $\mathsf{Fil}(\mathcal{C})$. Also, I should …
5
votes
Accepted
Criteria for being an $\infty$-category?
Since this example is kinda fun, let me spell it out. (The intuition should be clear though: the simplicial category I defined is really the result of taking a not-so-exciting (2,2)-category and apply …