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Homotopy theory, homological algebra, algebraic treatments of manifolds.

6 votes
Accepted

Waldhausen $K$-theory before group completion

I'm not sure about Waldhausen categories in general, but if you restrict attention to stable $\infty$-categories (with trivial Waldhausen structure in which all maps are cofibrations) then group compl …
Yonatan Harpaz's user avatar
5 votes

Criterion for alternation of the linking form

This is not an answer, but too long for a comment, and hopefully of some use. I believe the theorem you mention should be true for any 5-dimensional Poincaré duality space $X$. You can define the li …
j.c.'s user avatar
  • 13.6k
6 votes
Accepted

Spelling out explicitly the data of a two step filtration in terms of pieces and gluing data

Technically speaking the answer to your question is no, in the sense that the data of $(\alpha,\beta,\gamma,\delta)$ alone does not determine the filtered object $V_0 \subseteq V_1 \subseteq V_2$. How …
Yonatan Harpaz's user avatar
7 votes
Accepted

About fibrations with fibre Eilenberg-MacLane spaces

No. If this were the case then there would be a section $s: B \to E$ to $f$ induced by the $G$-equivariant map $\widetilde{s}:\widetilde{B} \to \widetilde{B} \times {\rm K}(M,n)$ sending $x$ to $(x,0) …
Yonatan Harpaz's user avatar
5 votes

Why is there a duality between spaces and commutative algebras?

I have not yet considered the new answer in full detail, so apologies for not addressing it. I'm coming back to this question after a while, so I thought I'd share some observations which came up duri …
Yonatan Harpaz's user avatar
49 votes
4 answers
4k views

Why is there a duality between spaces and commutative algebras?

1) The category of affine varieties over $\mathbb{C}$ is equivalent to the opposite category of finitely generated reduced algebras over $\mathbb{C}$. The equivalence associates to an affine variety i …
1 vote

Construction for algebras over little cubes operad

As you point out, it indeed seems that in order to get a well-behaved answer one should work in a suitable homotopical setting, for example, that of $\infty$-categories and $\infty$-operads. Using thi …
Yonatan Harpaz's user avatar
6 votes

Relation between the Hochschild cohomology of group algebras and groupoids

For every $\mathbb{Z}G$-bimodule $M$ there exists a $G$-module $U(M)$ such that the Hochschild cohomology of $\mathbb{Z}G$ with coefficients in $M$ is naturally isomorphic to the group cohomology of $ …
Yonatan Harpaz's user avatar
5 votes

Gray product on $(\infty,2)$-categories

For question (2), there is actually a left Quillen bifunctor $$ \times_{\mathrm{gr}}: \mathrm{Set}_\Delta^{\mathrm{sc}} \times \mathrm{Set}_\Delta^{\mathrm{sc}} \to \mathrm{Set}_\Delta^{\mathrm{sc}} $ …
Yonatan Harpaz's user avatar
26 votes
1 answer
1k views

Is the $\infty$-topos $Sh(X)$ hypercomplete whenever $X$ is a CW complex?

It can be shown (see Is every paracompact, Hausdorff, locally contractible space homotopy equivalent to a CW complex?) that if $X$ is a locally contractible paracompact Hausdorff space such that the $ …
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
7 votes

From relative categories to marked simplicial sets

Concerning the first question: the simplicial localization functor $L^H$ induces an equivalence from the relative category of small relative categories to the relative category of small simplicial cat …
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
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
9 votes
2 answers
1k views

Genuine equivariant ambidexterity

A particular case of Lurie and Hopkins' ambidexterity theory is that if $G$ is a finite group acting on a $K(n)$-local spectrum $X$ then the norm map $$ X_{hG} \to X^{hG} $$ is a $K(n)$-local equivale …

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