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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.

3 votes
Accepted

The locally model bicategory of $\cal V$-profunctors

Given two model categories $\mathcal{M},\mathcal{N}$, one does know what would have been a left Quillen functor out of what would have been the tensor product $\mathcal{M} \otimes \mathcal{N}$ into a …
Yonatan Harpaz's user avatar
8 votes
Accepted

Quillen equivalence, fibrant objects

Here is a counter-example to the dual assertion (so that you can get a counter-example to your original question by taking the opposite model categories). Consider the category ${\rm Set_\Delta}$ of s …
Yonatan Harpaz's user avatar
6 votes
Accepted

How is topological André-Quillen homology (TAQ) a "stabilization", exactly?

These stabilization formulas do indeed follow from the paper of Basterra-Mandell. Fix a commutative $S$-algebra $A$. Then Basterra and Mandell prove the following: 1) [Theorem 3] Given a commutative …
Yonatan Harpaz's user avatar
26 votes
Accepted

Why study the p-completions of a space?

First one should separate between the property and being $p$-complete and process of $p$-completion. In the classical setting, the $p$-completion functor is not so well-behaved for general spaces. For …
Yonatan Harpaz's user avatar
14 votes

Sheaves of complexes and complexes of sheaves

If $A$ is a Grothendieck abelian category then $Sh(X,A)$ is a Grothendieck abelian category, in which case one can endow the category $C(Sh(X,A))$ of unbounded complexes in $Sh(X,A)$ with the injectiv …
Yonatan Harpaz's user avatar
5 votes
Accepted

real and complex vector spaces as topological categories

I think the answer is no. Suppose there exists an enrichment satisfying your requirements, and let $U: Vect_{\mathbb{C}} \to Vect_{\mathbb{R}}$ be the forgetful functor. Let $C \subseteq Map(\mathbb{R …
Yonatan Harpaz's user avatar
5 votes
Accepted

Proposition in HTT on cofibrations of categories

You can argue as follows. Suppose that $g: D \to D'$ is a retract of $f: C \to C'$ (in the category of $S$-enriched categories) via maps $D \stackrel{i}{\to} C \stackrel{r}{\to} D$ and $D' \stackrel{i …
Yonatan Harpaz's user avatar
5 votes
Accepted

A finite Whitehead Theorem for $\infty$-topos

Let $\mathcal{X}$ be the $\infty$-topos in question containing an object $X \in \mathcal{X}$. I assume that by $X$ having homotopy dimension $\leq n$ you mean that the $\infty$-topos $\mathcal{X}_{/X} …
Yonatan Harpaz's user avatar
6 votes
Accepted

Compatibility of Grothendieck construction with pullback

Yes, though it is usually written as the commutativity of unstraightening with pullback (on the $\infty$-categorical level it doesn't matter, since straightening and unstraightening are inverse equiva …
Yonatan Harpaz's user avatar
15 votes

Why Grothendieck's Homotopy Hypothesis is so difficult?

First, let me remark that your question does not seem to be about the homotopy hypothesis, but about rectification. More specifically, the homotopy hypothesis concerns the question of whether (some ve …
Yonatan Harpaz's user avatar
13 votes
1 answer
474 views

Is the operadic nerve functor an equivalence of ∞-categories?

It is now known that the $\infty$-category of $\infty$-operads as defined by Lurie is equivalent to the underlying $\infty$-category of the model category of simplicial operads, see http://arxiv.org/p …
Yonatan Harpaz's user avatar
5 votes

Property-like structure in a model category

Specifically for the case of quasi-categories (or any other model for $\infty$-categories) the following observation can be useful: suppose that $f: {\cal C} \to {\cal D}$ is a map of quasi-categories …
Yonatan Harpaz's user avatar
7 votes
Accepted

Methods for defining/calculating homotopy limits of quasicategories

When working with quasi-categories, it is often more convenient (and more compatible with existing machinery) not to work with actual strict diagrams of quasi-categories but rather with coCartesian fi …
Yonatan Harpaz's user avatar
60 votes
Accepted

What is the mistake in the proof of the Homotopy hypothesis by Kapranov and Voevodsky?

Here is my guess. To compare spaces with their notion of strict $\infty$-groupoids (in which everything is strict except inverses) Kapranov and Voevodsky use an intermediate category of Kan diagrammat …
Yonatan Harpaz's user avatar
2 votes
Accepted

Morphisms of parametrized ring spectra

So the answer is a bit surprising (maybe I have a mistake). You have an adjunction between $Fun(X,\mathrm{Sp})$ and $\mathrm{Sp}$ which in one direction sends a functor to its (homotopy) colimit and o …
Yonatan Harpaz's user avatar

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