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Complex, contact, Riemannian, pseudo-Riemannian and Finsler geometry, relativity, gauge theory, global analysis.

7 votes
Accepted

Does $(M\times M, \omega\times -\omega)$ admit a Lagrangian fibration?

No (independent of the full meaning of the question) in general it does not. Any manifold which admits a Lagrangian fibration to a half dimensional base, also admits a Lagrangian foliation, and hence …
Thomas Kragh's user avatar
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1 vote
Accepted

If the second derivative of a function on $\mathbb R^n$ is everywhere nondegenerate, does it...

The counter example given in the comments by Brian Conrad (and Dylan Thurston) is very nice. However, it oscillates wildly at $\infty$, and I believe that if you assume some nice properties at $\infty …
Thomas Kragh's user avatar
  • 2,590
3 votes

Existence of Fermi coordinates on a Riemannian manifold

It is not enough to be within the injective radius. Let $(M,g)$ be an $(n+1)$-dimensional manifold and $\gamma\colon (-a,a) \to M$ a geodesic. One way to define Fermi coordinates $\phi \colon (-a,a)\ …
Thomas Kragh's user avatar
  • 2,590
5 votes

Diameter of a circle in an embedded Riemannian manifold

The answer to the first is NO. As mentioned by another answer you can use Do Carmo to prove a local result of the type: at points where the normal curvature is non-zero the 0-directions defines a fol …
Thomas Kragh's user avatar
  • 2,590
3 votes
0 answers
267 views

Maps of loop spaces with infinity-bounded differential.

I am currently working with loop spaces of manifold and finite dimensional manifolds approximating these and the following comes up very naturally: In the following piece-wise smooth means smooth on …
Thomas Kragh's user avatar
  • 2,590
6 votes

Are there topological obstructions to the existence of almost quaternionic structures on com...

It seems to me that if I understood the comments to my comment correctly that the map $$\mathrm{Sp}(1) \times \mathrm{Sp}(n) \to \mathrm{SO}(4n)$$ induced by right unit quarternionic multiplaction o …
Thomas Kragh's user avatar
  • 2,590
12 votes
3 answers
3k views

Are the stiefel-Whitney classes of the tangent bundle determined by the mod 2 cohomology?

Let $G=\mathbb{Z}/2\mathbb{Z}$. Let $f\colon L \to N$ be a smooth map of connected smooth closed $n$-dimensional manifolds such that the induced map $$f^* \colon H^*(N,G) \to H^*(L,G)$$ is an isomor …
Thomas Kragh's user avatar
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9 votes
0 answers
248 views

Parametrized cancelations in stable Morse theory

Let $B$ be a closed manifold. Let $\pi : M\to B$ be a submersion such that each fiber is a manifold without boundary. Let $f : M \to \mathbb{R}$ be a function such that the restrictions $f_x$ to each …
Thomas Kragh's user avatar
  • 2,590