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Complex geometry is the study of complex manifolds, complex algebraic varieties, complex analytic spaces, and, by extension, of almost complex structures. It is a part of differential geometry, algebraic geometry and analytic geometry.

10 votes

Converses to Cartan's Theorem B

For (4), there are finite CW complexes $X$ which are not contractible, but such that $H^q(X, \mathcal{F})$ vanishes for all (finite rank) locally constant sheaves $\mathcal{F}$. Specifically, take $X$ …
Dan Petersen's user avatar
  • 40.3k
13 votes
Accepted

Understanding the Hodge filtration

The naive Hodge filtration of a smooth affine variety is, indeed, the whole thing. We always have the short exact sequence of complexes: $$0 \to \Omega^{\bullet, \geq p} \to \Omega^{\bullet} \to \Omeg …
David E Speyer's user avatar
4 votes

Is this $\mathbb C$-fibration over compact Riemann surface trivial?

Maybe I misunderstood the question, but it seems to me that $M$ could be the total space of some other globally generated line bundle over $S$: For example, the total space of $\mathcal{O}(n)$ over $\ …
David E Speyer's user avatar
6 votes

Is a positive degree self map on a Riemann surface homotopic to a holomorphic self map?

The answer is no. This is a corrected version of Nicolast's comment. Let $E$ be an elliptic curve, let $f: E \to E$ be an endomorphism and let $H_1(f) : H_1(E) \to H_1(E)$ be the induced map on $H_1$. …
David E Speyer's user avatar
15 votes
Accepted

Relation between the cohomology group of a curve and the cohomology group of its jacobian

$\def\Alb{\text{Alb}}\def\Pic{\text{Pic}}\def\CC{\mathbb{C}}\def\ZZ{\mathbb{Z}}\def\RR{\mathbb{R}}\def\cO{\mathcal{O}}$There are two abelian varieties associated to a smooth projective connected $n$-f …
David E Speyer's user avatar
6 votes
Accepted

When Atiyah class and Chern class coincide?

$\def\ZZ{\mathbb{Z}}\def\CC{\mathbb{C}}\def\cO{\mathcal{O}}$To spell out my comment a little more, let $Z^1$ be the sheaf of $\partial$-closed holomorphic $(1,0)$-forms. Since "holomorphic" means $\ov …
David E Speyer's user avatar
8 votes
Accepted

Are smooth Schubert varieties Kähler?

Yes, smooth closed subvarieties of projective varieties are projective, and hence Kahler. Smooth Schubert varieties are very rare though, see the sources below for a description of them: Ryan, Kevin M …
David E Speyer's user avatar
7 votes

$H^{p,q}(X)$ versus $H^{q}(X, \bigwedge^p TX)$

One reason that $H^p(X, \bigwedge^q TX)$ will not be as well behaved as $H^p(X, \bigwedge T^{\ast} X)$ is that it is not deformation invariant, and thus not topological. In other words, if we have a c …
David E Speyer's user avatar
9 votes
Accepted

Can all $L^2$ holomorphic functions on a domain vanish at a particular point?

$\def\CC{\mathbb{C}}$Here is a less trivial example that I think works. Let $U \subset \CC^2$ be $$\{ (x,y) : |x| \leq \min(1, 1/|y|) \}$$ There are lots of $L^2$ holomorphic functions because the ch …
Martin Sleziak's user avatar
4 votes

Rational functions on reduced complex varieties that extend to global holomorphic functions

The answer is yes. Ariyan Javanpeykar has contributed the hard part; here are the easy parts. Let $\tilde{A}$ be the integral closure of $A$ in $\mathrm{Frac}(A)$ and let $\tilde{X} = \mathrm{Spec}(\ …
David E Speyer's user avatar
17 votes

Examples of smooth manifolds admitting inbetween one and a continuum of complex structures

There are countably many complex structures on $S^2 \times S^2$ up to isomorphism. Specifically, the even Hirzebruch surfaces $F_{2k}$ are the only options. This is the main result of Qin, Zhenbo, C …
David E Speyer's user avatar
10 votes
Accepted

The Construction of a Basis of Holomorphic Differential 1-forms for a given Planar Curve

I'm getting back to the question of describing holomorphic $1$-forms on a plane curve. Affine curves: Let $X$ be a smooth curve in $\mathbb{A}^2$, given by the equation $F(x,y)$. Then $F_x dx + F_y …
David E Speyer's user avatar
10 votes

The Construction of a Basis of Holomorphic Differential 1-forms for a given Planar Curve

$\def\CC{\mathbb{C}}$I'll come back later and leave an answer to (1), which is classical and straightforward. I have nothing to say about (3). I thought the most interesting question was (2), but I am …
David E Speyer's user avatar
4 votes
Accepted

Cotangent bundle of coadjoint orbit is stein manifold?

Not with the obvious complex structure. Notice that $O_{\lambda}$ is a closed subvariety of $T^{\ast}(O_{\lambda})$ (namely the zero section). Closed subvarieties of Stein varieties are Stein. However …
David E Speyer's user avatar
6 votes

Are holomorphic vector bundles over Kähler manifolds Kähler

Proposition 3.18 of Voisin's Hodge Theory and Complex Algebraic Geometry I says that, if $X$ is compact Kahler and $E$ is a holomorphic vector bundle over $X$, then $\mathbb{P}(E)$ is Kahler. Since $E …
David E Speyer's user avatar

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