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

27 votes

"The complex version of Nash's theorem is not true"

As Faisal says, there is no hope to have a general Nash-type theorem for all complex manifold, when the ambient space considered for the (isometric) embedding is some $\mathbb C^N$: no compact complex …
diverietti's user avatar
  • 7,902
27 votes
Accepted

Is the complex structure of $\mathbb CP^n$ unique?

Let me write this too long comment as an answer. As abx says, what we do know is Theorem 1. If a Kähler manifold $X$ is homeomorphic to $\mathbb{CP}^n$, then $X$ is biholomorphic to it. This is due to …
diverietti's user avatar
  • 7,902
22 votes
1 answer
1k views

Relationship between the signs of different notions of curvature in complex geometry

Let $(X,\omega)$ be a complex hermitian manifold, and call $\Theta$ its Chern curvature tensor. Out of this we can consider different notions of curvature, namely the holomorphic bisectional curvature …
diverietti's user avatar
  • 7,902
15 votes

Non-Kahler Complex manifolds

Let $X$ be a compact complex manifold of complex dimension $n$. The Hodge-Frölicher spectral sequence starts with $$ E_1^{p,q}=H^{p,q}(X,\mathbb C) $$ and the limit term $E^{p,q}_\infty$ is the grade …
diverietti's user avatar
  • 7,902
12 votes
Accepted

Relation between the de Rham and Hodge Laplacians on the Exterior Algebra

If $(X,\omega)$ is Kähler, then it is always true that $$ \Delta'=\Delta''=\frac 12\Delta, $$ where these three Laplacians are with respect, in order, to $\partial$, $\bar\partial$ and $d$. This is va …
diverietti's user avatar
  • 7,902
12 votes
2 answers
2k views

Negative holomorphic sectional curvature

Let X be a complex hermitian manifold with hermitian form $\omega$. How can you prove that if $\omega$ has negative holomorphic sectional curvature, then its scalar curvature is negative, too?
diverietti's user avatar
  • 7,902
9 votes
Accepted

Negative holomorphic sectional curvature

Here is the answer. Let $(X,\omega)$ be a Kähler $n$-dimensional manifold. Fix a point $x_0\in X$ an choose local holomorphic coordinates $(z_1,\dots,z_n)$ centered at $x_0$ and such that $(\partial/ …
diverietti's user avatar
  • 7,902
9 votes
Accepted

Differential forms on an almost complex manifold

Call $C^{\infty}_{p,q}(M)$ the space of smooth complex sections of the bundle $\Lambda^{p,q}T^*_M$ and let $2n$ be the real dimension of $M$. The fact that $$ dC^{\infty}_{p,q}(M)\subset C^{\infty}_ …
diverietti's user avatar
  • 7,902
9 votes

On the generalized Gauss-Bonnet theorem

Maybe what follows is not exactly what you are looking for, but it gives you an answer at least when you don't want to restrict yourself to the tangent bundle and work rather with general (complex) ve …
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  • 7,902
8 votes

Positive (k.k)-form?

I will try to clarify a little bit your question and at the same time to give you an answer. All that I say, you can find on Demailly's book "Complex Analytic and Differential Geometry". GENERAL THEO …
diverietti's user avatar
  • 7,902
7 votes

Griffiths-positive metric

As soon as the base manifold has dimension greater than one, the existence of Griffiths positive metrics on an ample vector bundle is not known (and even in the one dimensional case, it is not obvious …
diverietti's user avatar
  • 7,902
7 votes

Hom between Brody hyperbolic varieties

I assume that for $\operatorname{Hom}(X,Y)$ you mean $\operatorname{Hol}(X,Y)$, that is the family of all holomorphic maps from $X$ to $Y$, endowed with its universal complex structure (which exists s …
diverietti's user avatar
  • 7,902
5 votes

Is hyperbolicity a Zariski open condition?

Kobayashi hyperbolicity (or Brody hyperbolicity, the two notions coincide for compact complex spaces), is an open condition with respect to the analytic topology. Thus, for instance, once you find an …
diverietti's user avatar
  • 7,902
5 votes
Accepted

Holonomy of a Kähler manifold

The answer is yes. The holonomy principle states that a given a riemannian manifold $(M,g)$ and a point $x\in M$, the datum of a parallel tensor field of a given type is equivalent to the datum of a …
diverietti's user avatar
  • 7,902
5 votes

A question on Ricci curvature and Ricci form.

The whole point is that if the metric is Kähler, then the Chern connection coincides with the complexification of Levi-Civita connection of the riemanian metric on the underlying real manifold given b …
diverietti's user avatar
  • 7,902

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