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

0 votes

Can you have a spherical plane?

I don't think so: A surface of constant positive curvature is diffeomorphic to a sphere, so it would be compact. If the identity map were continuous, then by composition the image of the surface woul …
Gunnar Þór Magnússon's user avatar
4 votes

Wanted: an example of a natural non-K\"ahler metric on a Kahler manifold

Thanks Claudio, Francesco and José for your interesting answers. This is an "answer in absence" of Demailly; he doesn't use this site, but I thought his remark was nice enough to share. What follows …
Gunnar Þór Magnússon's user avatar
9 votes
1 answer
1k views

Calculating a second fundamental form in the space of hermitian metrics

Let $X$ be a compact Kahler manifold and let $\mathcal M$ denote the space of hermitian metrics on $X$. We'll identify a hermitian metric with a smooth, real and positive $(1,1)$-form $\omega$. Let $\ …
Gunnar Þór Magnússon's user avatar
16 votes
4 answers
1k views

Wanted: an example of a natural non-Kähler metric on a Kähler manifold

Let $X$ be a Kähler manifold. Associated to any hermitian metric $h$ on $X$ is a smooth real $(1,1)$-form $\omega = -\text{Im } h$, called the Kähler form of $h$. One of several equivalent conditions …
Gunnar Þór Magnússon's user avatar
3 votes
Accepted

Almost Complex Integrability and Algebraic Varieties

If you want equivalent conditions to the Nijenhuis tensor vanishing then one is that the induced $\bar \partial$ operator defines a complex, i.e. that $\bar \partial^2 = 0$. Another one is that the ex …
Gunnar Þór Magnússon's user avatar
0 votes

Are there hermitian metrics with the volume form of a Kahler metric?

I really should think more about these things before asking. The answer is "yes". K. Yang considers the flag manifold $F := F_{1,2,3} := SU(3)/S(U(1)^3)$ in Invariant Kahler metrics and projective em …
Gunnar Þór Magnússon's user avatar
5 votes
Accepted

What does the Kähler cone of the one-point blow-up of $\mathbb{C}P^n$ look like?

The Kahler cone of any compact manifold is described by a theorem of Demailly and Paun. If $X$ is a compact Kahler manifold, then its Kahler cone is one of the connected components of the set $$ \math …
Gunnar Þór Magnússon's user avatar
4 votes
Accepted

Examples of non-Kahler surfaces with explicit non-Kahler metric

If your surface is fairly explicit you can obtain an explicit hermitian metric on it as well. For example, if we take Francesco's Hopf surface $X$, then a hermitian metric $\omega$ on $X$ can be ident …
Gunnar Þór Magnússon's user avatar
6 votes
1 answer
1k views

Harmonic forms on Ricci-flat Kahler manifolds

Let $X$ be a compact Kahler manifold with $c_1(X) = 0$. Any Kahler metric $\omega$ on $X$ gives a Laplacian $\Delta_\omega$ and the $(1,1)$-form $\omega$ is harmonic with respect to this Laplacian. …
Gunnar Þór Magnússon's user avatar
2 votes
1 answer
281 views

Is a certain composition of harmonic forms again harmonic?

Let $(X,\omega)$ be a compact Kahler manifold, and let $\alpha$ and $\beta$ be smooth $(1,1)$-forms on $X$ that are harmonic (with respect to $\omega$). I can consider each of my $(1,1)$-forms as an a …
Gunnar Þór Magnússon's user avatar
3 votes
1 answer
461 views

Software for calculating products and sums of Kronecker deltas

I am looking at a Kahler metric $g$ on a certain manifold $M$, which has the good taste to be invariant under a transitive group of isometries, and I want to say something about its holomorphic sectio …
Gunnar Þór Magnússon's user avatar
2 votes
2 answers
898 views

Are there hermitian metrics with the volume form of a Kahler metric?

Let $X$ be a compact Kahler manifold of complex dimension $n$. The Aubin--Calabi--Yau theorem says that if we fix a smooth form $\rho$ in the Chern class $c_1(X)$, then every Kahler class on $X$ conta …
Gunnar Þór Magnússon's user avatar
5 votes
1 answer
570 views

Translation of Kähler's "Über eine bemerkenswerte Hermitesche Metrik"

Has anyone translated Erich Kähler's "Über eine bemerkenswerte Hermitesche Metrik" into English or French? (Preferably, but I'll take anything.)
Gunnar Þór Magnússon's user avatar
13 votes
1 answer
2k views

Surgery in complex geometry

I've been thinking about surgery on complex manifolds. Not very seriously, but just to the point that I think it's odd how there's almost no mention of it in the literature. I figure there's something …
Gunnar Þór Magnússon's user avatar
33 votes
2 answers
6k views

Which almost complex manifolds admit a complex structure?

I was reading Yau's list of problems in geometry, and one of them is to prove that any almost complex manifold of complex dimension $n \geq 3$ admits a complex structure. It's been some time since Yau …
Gunnar Þór Magnússon's user avatar

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