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Homotopy theory, homological algebra, algebraic treatments of manifolds.

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
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 …
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
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 $ …
Yonatan Harpaz's user avatar
19 votes
3 answers
2k views

Is every paracompact, Hausdorff, locally contractible space homotopy equivalent to a CW comp...

Milnor proved that any paracompact Hausdorff space which is equi-locally convex (and hence in particular locally contractible) is homotopy equivalent to a CW complex. However, unlike being paracompact …
Yonatan Harpaz's user avatar
18 votes
Accepted

Is every paracompact, Hausdorff, locally contractible space homotopy equivalent to a CW comp...

After some more digging I found a (somewhat non-explicit) counterexample to the original question. In his paper "un espace metrique lineaire qui n’est pas un retracte absolu" Cauty constructs a metric …
Yonatan Harpaz's user avatar
10 votes

Is every paracompact, Hausdorff, locally contractible space homotopy equivalent to a CW comp...

It seems that at least a partial answer can be given using the formalism of $\infty$-topoi: if $X$ is a paracompact Hausdorff space which is locally contractible (in the strong sense discussed above) …
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 …
Yonatan Harpaz's user avatar
9 votes
Accepted

Is every locally compactly generated space compactly generated?

The paper "A distinguishing example in k-spaces" by John Isbell constructs an example of a locally compact space $X$ which is not compact-Hausdorffly generated.
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
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
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
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
6 votes

Which sequential colimits commute with pullbacks in the category of topological spaces?

Fiber products do commute with sequential colimits of closed embedding in CGWH, but one must remember that neither limits nor colimits in CGWH are the same as the corresponding limits and colimit in t …
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

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