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Homotopy theory, homological algebra, algebraic treatments of manifolds.

14 votes
3 answers
1k views

Is intersection homology the usual homology of something else?

Let $X$ be the sort of topological space for which it makes sense to talk about the intersection homology. Fix a perversity $p$, or just take $p= 1/2$ if you like. Is there some naturally d …
Vivek Shende's user avatar
  • 8,723
6 votes
1 answer
264 views

Which manifolds are a sphere bundle in more than one way?

Certain spheres admit nontrivial fibrations, i.e. the Hopf fibrations and the maps to projective spaces. Also, a product of spheres is a sphere bundle in more than one way. Are there manifolds …
Vivek Shende's user avatar
  • 8,723
15 votes
0 answers
588 views

What is the determinant of Poincaré duality?

For a complex $C^{\bullet}$ of finite dimensional vector spaces, one has a determinant $$|C^\bullet|:= \bigotimes \left(\Lambda^{top} C^i\right)^{(-1)^i}$$ functorial with respect to quasi-isomorphi …
Vivek Shende's user avatar
  • 8,723
5 votes

Is the Euler characteristic of a certain nonlinear variety related to that of a certain line...

Such a variety always has Euler number zero. This is because: Due to being cut out by homogenous equations, it is invariant under the $\mathbf{C}^*$ action $t.(x_1,\ldots,x_n) \to (tx_1, \ldots, t …
Vivek Shende's user avatar
  • 8,723
1 vote

Genera and the Milnor Conjecture on the Unknotting Number of a Torus Knot

It is a bit unclear what you are asking. The thing you call the geometric genus is certainly not the geometric genus; the literal object you wrote is $\infty$ and if you first compactified the curve …
Vivek Shende's user avatar
  • 8,723
8 votes
1 answer
1k views

Euler characteristics and characteristic classes for real manifolds?

Consider an oriented manifold $X$. To calculate its Euler characteristic, one might integrate the Euler class. Now if $X$ were a complex manifold, and given as a section of some complex bundle $E$ o …
Vivek Shende's user avatar
  • 8,723
8 votes
0 answers
335 views

What's the Hochschild homology of the category of constructible sheaves?

Let $X$ be a manifold. Does the Hochschild homology/cohomology of the category of constructible sheaves on $X$ have a more familiar name?
Vivek Shende's user avatar
  • 8,723
11 votes
1 answer
953 views

Chern numbers via Euler characteristics?

Let $X$ be a space good enough to have a fundamental class, and $E$ a complex vector bundle on $X$. Let $P$ be some polynomial expression, and say I want to evaluate $P(c_i(E)) \cap [X]$. Is the …
Vivek Shende's user avatar
  • 8,723
16 votes
Accepted

What is the Euler characteristic of a Hilbert scheme of points of a singular algebraic curve?

For singular plane curves, there is a conjectural formula (due to Alexei Oblomkov and myself) in terms of the HOMFLY polynomial of the links of the singularities. For curves whose singularities are t …
Vivek Shende's user avatar
  • 8,723
14 votes
0 answers
411 views

Does the category of G-spectra know G?

I was recently in the situation of having access to the category of $G$-modules (for some group $G$ which I had forgotten), as just a category, i.e. no monoidal structure, together with the forgetful …
Vivek Shende's user avatar
  • 8,723
11 votes
1 answer
547 views

Measure the failure of colimit to commute with taking free loops (or Hochschild homology)?

For a space1 $X$, let $\mathcal{L}X = \mathrm{Maps}(S^1, X)$ be the free loop space. Inclusion of constant loops gives a natural map $X \to \mathcal{L}X$. This is not a homotopy equivalence unless $X …
Vivek Shende's user avatar
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