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For questions involving one or more categorical dimensions, or involving homotopy coherent categorical structures.

8 votes
0 answers
298 views

Can Postnikov towers converge without Postnikov completeness?

In Higher Topos Theory, Lurie says that "Postnikov towers are convergent" in a presentable $\infty$-category $\mathcal{C}$ if $\mathcal{C}$ is equivalent to the $\infty$-category $\mathrm{Post}(\mathc …
12 votes
Accepted

Taking the category of sheaves is symmetric monoidal

Provided at least one of $M$ and $N$ is locally compact, the $\infty$-topos $\mathrm{Sh}(M \times N)$ is the product of $\mathrm{Sh}(M)$ and $\mathrm{Sh}(N)$ in $\mathrm{RTop}$. This is HTT 7.3.1.11. …
Reid Barton's user avatar
  • 25.2k
3 votes

The Grothendieck plus construction for stacks of n-types

This is discussed in section 3.4.3 of https://arxiv.org/abs/2004.00731 by Mathieu Anel and Chaitanya Leena Subramaniam.
Reid Barton's user avatar
  • 25.2k
6 votes
Accepted

A few questions while reading Higher Topos Theory

$\newcommand{\SSet}{\mathsf{SSet}}\DeclareMathOperator{\Map}{Map}$First, let's record the fact that for any $A$ in $\SSet_{/S}$ and any right fibration $p : X \to S$, the simplicial set $\Map_{\SSet_{ …
Reid Barton's user avatar
  • 25.2k
9 votes
2 answers
569 views

Are bicategories of lax functors also bicategories of of pseudofunctors?

Let A be a bicategory. Can I construct in some "natural" way a bicategory L(A) such that for every bicategory B, the bicategory of lax functors Lax(A, B) is (bi)equivalent to the bicategory of pseudo …
9 votes
Accepted

Homotopy Pushouts via Model Structure in Top

Question 1: The model category $\mathcal{C}$ should be left proper, i.e. the pushout of a weak equivalence along a cofibration is again a weak equivalence. (Dually, there is a notion of right proper. …
David White's user avatar
  • 30.3k
16 votes
2 answers
2k views

What's an example of an "adjunction up to adjunction"?

(There are several dialects of 2-categorical language. Mine is the one where everything is weak by default. You might need to insert the word "weak" or "pseudo" (or even "strong", if your default is …
5 votes
Accepted

What functor is adjoint to the tensor product of 2-vector spaces?

I'll denote your category of 2-vector spaces by 2Vect. By your preliminary remarks, 2Vect is actually the category of Vect-valued presheaves on Δ≤1 where Δ≤1 denotes the full subcategory of Δ on the …
Reid Barton's user avatar
  • 25.2k
9 votes
2 answers
1k views

Terminology: lax vs. oplax colimits

I would like to know the standard usage of "lax colimit" and "oplax colimit" in the 2-categorical literature. The nLab does not give an explicit definition of "lax colimit", as far as I can see, and …
9 votes
Accepted

$(\infty,1)$-categories and model categories

Mostly I refer you to my answer here and also this question. To answer the question about (co)fibrations: No, there is no notion corresponding to (co)fibration in the (∞,1)-category associated to a m …
Community's user avatar
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7 votes
Accepted

Is every left fibration of simplicial sets with nonempty fibers a trivial kan fibration?

The inclusion $\partial \Delta^n \times \Delta^1 \subseteq X(n+1)$ isn't any kind of anodyne extension, though. It's formed by attaching an n-simplex to $\partial \Delta^n \times \Delta^1$ with bound …
Reid Barton's user avatar
  • 25.2k
5 votes
Accepted

Models for, and motivation for, (oo,n)-categories for general n

For me a "model of (∞,n)-categories" is something (e.g., a model category) from which one can extract "the" (∞,1)-category of (∞,n)-categories. One could make this more precise by choosing a preferre …
Community's user avatar
  • 1
5 votes
Accepted

What are natural transformations in 1-categories?

Here is a counterexample for your next-to-last question. Let S be a set with more than one element and consider the two full subcategories of Cat on, respectively, the single category which is the di …
Reid Barton's user avatar
  • 25.2k
7 votes

(infinity,1)-categories directly from model categories

You might want to take a look at the responses to How to think about model categories? Even if you only cared about presentable (∞,1)-categories, you still might prefer the general (∞,1)-categorical …
Community's user avatar
  • 1
7 votes

Is there a meaningful difference between biased and unbiased composition?

Certainly unbiased definitions are the norm in modern homotopy theory. I guess an example of a biased definition is the (original?) definition by Stasheff of an $A_\infty$ space—the homotopy theorist …
Reid Barton's user avatar
  • 25.2k

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