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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 …
Dylan Wilson's user avatar
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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 …
Dylan Wilson's user avatar
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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 …
Dylan Wilson's user avatar
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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 …
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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 …
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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 …
Dylan Wilson's user avatar
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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 …
Dylan Wilson's user avatar
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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: …
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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 …
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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 …
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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 …
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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) …
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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 …
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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 …
Dylan Wilson's user avatar
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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 …
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