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Topology of cell complexes and manifolds, classification of manifolds (e.g. smoothing, surgery), low dimensional topology (e.g. knot theory, invariants of 4-manifolds), embedding theory, combinatorial and PL topology, geometric group theory, infinite dimensional topology (e.g. Hilbert cube manifolds, theory of retracts).

9 votes
3 answers
3k views

Does a connected manifold with vanishing Euler characteristic admit a nowhere-vanishing vect...

A version of the "hairy ball" theorem, due probably to Chern, says that the Euler-characteristic of a closed (i.e. compact without boundary) manifold $M$ can be computed as follows. Choose any vector …
Theo Johnson-Freyd's user avatar
5 votes
1 answer
538 views

What is the Heegaard Floer Homology of a connect sum of $S^2 \times S^1$s?

There are many conjecturally-equivalent three-manifold Floer homologies, of which my understanding is the most-computable is Heegaard Floer homology. What is the (Heegaard) Floer homology of a c …
Theo Johnson-Freyd's user avatar
4 votes
2 answers
539 views

Is every group object in TopMan a Lie group?

Recall that a Lie group is a group object in the category of C∞ manifolds. If I have a group object in the category of topological manifolds, can I necessarily equip it with a smooth structure so tha …
Theo Johnson-Freyd's user avatar
16 votes
2 answers
771 views

Are there results from gauge theory known or conjectured to distinguish smooth from PL manif...

My question begins with a caveat: I sometimes spend time with topologists, but do not consider myself to be one. In particular, my apologies for any errors in what I say below — corrections are encou …
Theo Johnson-Freyd's user avatar
5 votes
1 answer
687 views

What is the Euler characteristic of a mapping space?

Suppose that $A$ and $B$ are topological spaces homotopy equivalent to finite cell complexes, and let $B^A = \mathrm{maps}(A,B)$ denote the space of maps from $A$ to $B$. Is it there a formula for th …
Theo Johnson-Freyd's user avatar
94 votes
4 answers
15k views

Can every manifold be given an analytic structure?

Let $M$ be a (real) manifold. Recall that an analytic structure on $M$ is an atlas such that all transition maps are real-analytic (and maximal with respect to this property). (There's also a sheafy …
Theo Johnson-Freyd's user avatar
4 votes
1 answer
352 views

When does a VBLA induce an isomorphism on Lie algebroid cohomology?

This question is geared towards the experts, so I will only briefly gloss the definitions. Everything I say is in the category of finite-dimensional smooth manifolds, and whenever I say "$\mathbb Z$- …
Theo Johnson-Freyd's user avatar
19 votes
1 answer
1k views

How difficult is Morse theory on stacks?

The title is a little tongue-in-cheek, since I have a very particular question, but I don't know how to condense it into a pithy title. If you have suggestions, let me know. Suppose I have a Lie gro …
Theo Johnson-Freyd's user avatar
10 votes
2 answers
700 views

When does an even-dimensional manifold fiber over an odd-dimensional manifold?

Are there simple necessary and sufficient conditions for an (oriented) even-dimensional compact smooth manifold to fiber over an (oriented) odd-dimensional manifold (with oriented fibers)? For ex …
Theo Johnson-Freyd's user avatar
5 votes
0 answers
166 views

In cell-decomposed manifolds, how easy is it to arrange for the tubular neighborhood of a di...

Suppose that you have decomposed a manifold $M$ into cells (I care most, if it matters, about compact oriented smooth manifolds; but if my question can be solved in the PL category, all the better). …
Theo Johnson-Freyd's user avatar
24 votes
5 answers
3k views

Can surfaces be interestingly knotted in five-dimensional space?

It's possible this question is trivial, in which case it will be answered quickly. In any case, I realized that it's a basic question the answer to which I should know but do not. Everybody loves kn …
Theo Johnson-Freyd's user avatar
6 votes
1 answer
440 views

Are framed manifolds cubulatable?

Let's say an $n$-manifold is cubulated if it is glued out of cubes $[0,1]^n$ in a way that looks locally like the standard cubulation of $\mathbb R^n$. For instance, the face $[0,1]^{k-1} \times \{1\} …
Theo Johnson-Freyd's user avatar
18 votes
1 answer
515 views

How aggressive is the fibrant replacement of $\mathrm{Bord}_n$?

Lurie (On the Classification of Topological Field Theories), with some corrections by Calaque and Scheimbauer (A note on the $(\infty,n)$-category of cobordisms), famously constructed a symmetric mono …
Theo Johnson-Freyd's user avatar
11 votes
1 answer
334 views

Reference request: sheaves on the site of d-manifolds

I believe I know how to prove the following results. I also know to whom to cite fancy-shmancy results that have these as a very special case. My question is: what are the correct citations for thes …
Theo Johnson-Freyd's user avatar