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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.

41 votes
Accepted

de Rham vs Dolbeault Cohomology

Let $\Omega^{p,q}(M)$ be the $C^{\infty}$ $(p,q)$-forms. One always has a double complex with $\Omega^{p,q}(M)$ in position $(p,q)$. The cohomology in the $q$ direction is Dolbeault cohomology, the co …
David E Speyer's user avatar
22 votes
Accepted

Are all holomorphic vector bundles on a contractible complex manifold trivial?

No, even for line bundles. We have the short exact sequence of sheaves $$0 \to \underline{\mathbb{Z}} \overset{2 \pi i}{\longrightarrow} \mathcal{O} \overset{\exp}{\longrightarrow} \mathcal{O}^{\ast} …
David E Speyer's user avatar
19 votes
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Intuitive Aproach to Dolbeault Cohomology

Here is some nonsense that I find useful: On a complex manifold, $$\frac{\mbox{locally constant functions}}{\mbox{smooth functions}} \approx \frac{\mbox{locally constant functions}}{\mbox{holomorphic …
David E Speyer's user avatar
18 votes
Accepted

$\partial \bar{\partial}$ lemma for contractible domains

Okay, here is a counter-example. Let $X$ be the following open subset of $\mathbb{C}^2$: $$X:= \{ (z_1, z_2) : |z_1| < 2,\ |z_2| < 1 \} \cup \{(z_1, z_2): |z_1| < 1,\ |z_2| < 2 \}$$ This is the stand …
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
15 votes
Accepted

A topological consequence of Riemann-Roch in the almost complex case

I believe that the displayed equation is valid for almost complex manifolds. This is closely related to a computation I talked about here. Let $r_1$, $r_2$, ..., $r_n$ be the chern roots of the tange …
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
14 votes
Accepted

Algebraic de Rham cohomology vs. analytic de Rham cohomology

I don't think you can get this directly from GAGA. The reference that I know for this result is Grothendieck, On the de Rham cohomology of algebraic varieties. It is short, beautiful, and in English.
David E Speyer's user avatar
13 votes
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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
13 votes
Accepted

Hard Lefschetz Theorem for the Flag Manifolds

I'll spell out Hard Lefschetz as an explicit combinatorial statement about the Grassmannian $G(d,n)$. I can definitely give you a version of this for the full flag manifold if you want it and probably …
David E Speyer's user avatar
12 votes
Accepted

Nonalgebraic complex manifolds

I wrote a blog post about some of the standard examples of nonalgebraic compact complex manifolds.
David E Speyer's user avatar
10 votes

Why are local systems on a complex analytic space equivalent to vector bundles with flat con...

You might also want to read Carlos Simpson's paper "Moduli of representations of the fundamental group of a smooth projective variety", parts I and II. He explains in great detail how to make the set …
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
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 $X$ …
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 …

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