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Homotopy theory, homological algebra, algebraic treatments of manifolds.

1 vote

Singular analog of cellular homology

$\require{AMScd}$ Let $X$ be a topological space. Here is something that might deserve to be called the singular CW complex of $X$. Let us call it $CW(X)$. It is constructed inductively by skeletons. …
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
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
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
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 …
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
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
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
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
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

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
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
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

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

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