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Complex, contact, Riemannian, pseudo-Riemannian and Finsler geometry, relativity, gauge theory, global analysis.
10
votes
Smooth manifolds that don't admit a partition of unity
In my experience the answer is not really.
My favorite example of a Hausdorff manifold which is not paracompact (which incidentally is also my all-time favorite counter example to most point-set topo …
3
votes
Two definitions of Calabi-Yau manifolds
I believe that the answer is yes, this needs Yau's theorem. Rather, I think that the statement of your question can be phrased in such a way as to be equivalent to Yau's theorem.
My understanding of …
2
votes
Detecting nonorientability
For certain $F$ and $\epsilon_i$ the answer is no. But it is probably yes for generic choices.
Here is an example with n=2 and $M$ the Klein bottle. We start with F being a standard projection of the …
35
votes
Accepted
Maps inducing zero on homotopy groups but are not null-homotopic
Consider ordinary singular cohomology with varying coefficients. You can look at the short exact sequence of abelian groups:
$$0 \to \mathbb{Z}/2 \to \mathbb{Z}/4 \to \mathbb{Z}/2 \to 0$$
This give …
1
vote
Are there Ricci-flat riemannian manifolds with generic holonomy?
The Ricci curvature is a local quantity, so I am only going to focus on the case that the local holonomy group is SO(n). Philosophically, the local holonomy group and the curvature of a connection att …
2
votes
Integration in equivariant K-theory
It sounds like what you are after is a theory of genuine equivariant K-theory (Which exists!)
If you think of a cohomology theory as a sequence of functors to abelian groups together with some proper …
4
votes
Spin structures on 7-dimensional spherical space forms
Here is a partial answer. If the order of $\Gamma$ is odd, then this is a trivial application of transfer maps. You have described your manifold as a quotient $\pi:S^7 \to M = S^7/\Gamma$, and hence $ …
7
votes
1
answer
497
views
Are holomorphic Lagrangians locally graphs?
Let $(M, \omega)$ be a holomorphic symplectic manifold of (complex) dimension $2n$. Let $x$ be a point in $M$. My understanding from the discussion and answers to this MO question is that there exists …
3
votes
Is there a complex structure on the 6-sphere?
This is a famous open-problem. It is still unknown.
143
votes
Accepted
What is torsion in differential geometry intuitively?
The torsion is a notoriously slippery concept. Personally I think the best way to understand it is to generalize past the place people first learn about torsion, which is usually in the context of Rie …
2
votes
Accepted
Relation between Morse Theory and integration against Euler Characteristic
I glanced at the paper briefly. Let me try to explain what I understand. Let us suppose that M is compact and without boundary. Let $f: M \to \mathbb{R}$ be a Morse function. Let us further suppose fo …
3
votes
1
answer
547
views
Is the Action of the mapping class group transitive on embedded arcs?
Let S be a surface of genus g with some parked points (n of them). Assume $n \geq 2$ and fix two of the marked points. Consider the set of embedded arcs going between these two special points. The gro …
8
votes
3
answers
1k
views
Derivations of C(X)? or Why Must Supermanifolds be Smooth?
What are the derivations of the algebra of continuous functions on a topological manifold?
A supermanifold is a locally ringed space (X,O) whose underlying space is a smooth manifold X, and whose …
7
votes
Accepted
Critical points on a fiber bundle
The part I'm still hesitant about is that the manifold you call $C$ is $n$-dimensional. Codimension arguments in the infinite dimensional setting are always a little sticky. So, I'm just going to trea …
16
votes
Accepted
Super-cobordisms
There are a number of technical issues with making what you describe precise, for example: what precisely is a supermanifold with boundary? how can you glue/compose bordisms? etc. I am going to ignore …