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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.

4 votes
0 answers
252 views

Zariski connectedness theorem: from analytic & topological viewpoint

Let $p:Y \to X$ be a proper, flat (see later why) surjective map between smooth connected complex varieties $Y,X$, esp. $X$ unibranch. Assume that there exist a Zariski open (esp. dense) $U \subset X$ …
-1 votes
1 answer
216 views

Almost Complex Structure extending to Complex Structure, aka "Integrable"

Let $M$ be a smooth manifold of (real) dimension $2n$. An almost complex structure $J$ on $M$ is a linear vector bundle isomorphism $J \colon TM\to TM$ on the tangent bundle $TM$ such that $J^2 = − 1 …
0 votes
0 answers
72 views

Lefschetz Theorem in Dolgachev's On automorphisms of Enriques Surfaces

Let $F$ be a Enriques surface over $\Bbb C$. I have a question about a detail in the proof of Proposition 2.1. from Dolgachev's On automorphisms of Enriques surfaces. This 2.1. Proposition. states th …
2 votes
0 answers
183 views

Zariski Connectedness Theorem in Complex Geometry

Let $f: X \to Y$ be a proper surjective morphism of complex irreducible varieties such that general fibre of $f$ is connected and $Y$ integrally closed\normal. Say, we even assume wlog $Y=\text{Spec}( …
2 votes
0 answers
164 views

Principle of degeneration as precursor of Zariski's connectedness theorem (geometric intuition)

I have following question about so-called "principle of degeneration" in algebraic geometry (which in modern terms is an immediate consequence of Zariski's main theorem and goes in it's original form …
2 votes
1 answer
212 views

Prefactor $2\pi i$ for Tate-Hodge structure

A rather basic question. What was the original reason to consider the underlying $\mathbb{Z}$-module of the - as canonical object regarded - Tate-Hodge structure $\mathbb{Z}(1)$ to be given as $2 \pi …
0 votes
1 answer
322 views

Self-intersection of zero section of line bundle over elliptic base curve

Let $C$ be an elliptic curve over $k=\mathbb{C}$ and $\mathcal{L}$ a line bundle of degree $d$. It induces naturally a $\mathbb{A}^1$-fibration $L \to C$ where $L=\underline{\operatorname{Spec}} (\ope …
2 votes
1 answer
350 views

Comparison of classical and Zariski topologies with constructible sets

In David Mumford's book Algebraic Geometry I, Complex Projective Varieties the proof of (3.25) Specialization principle on page 53 contains an argument I not understand. General assumptions: all our v …
2 votes
1 answer
133 views

Generically finite projection $\pi_L: X \to \mathbb{P}^2$ from plane $L$ and critical points

(In following we are working in "classical" complex setting: i.e. all involved schemes are considered to be varieties over $k=\mathbb{C}$) Let $X \subset \mathbb{P}^n$ be irreducible surface and $L $ …
2 votes
1 answer
594 views

Example motivating mixed Hodge structures

The suggested intuition behind mixed Hodge structures - developed in particular to generalize Hodge decomposition of cohomology groups from complex smooth complete varieties to more general algebraic …
3 votes
0 answers
220 views

Historical proof of Leschetz Hyperplane Theorem

I browse in Phillip Griffiths' Slides on historical development of Hodge-theory and these include a sketch of the original approach with Lefschetz used to study complex surfaces in his famous hypersur …
0 votes
0 answers
201 views

Expansion around a singular point of a multivalued meromorphic function (due to Riemann/Cauchy)

In Riemann's publication about Abelian functions 'Theorie der Abelschen Functionen' (Here the original paper in german) at the end of Chapter 4, part 2 is clamed that for every Riemann surface $T$ and …
0 votes
0 answers
290 views

Chow's Lemma: Mumford's and Grothendieck's (?) definitions

David Mumford gives in his book Algebraic Geometry I, Complex Projective Varieties on page 61 a definition of Chow's Lemma which has at least for me not a usual form: If says that a closed $^*$-analyt …
2 votes
0 answers
522 views

Polarizations in algebraic and symplectic geometry

In context of Abelian varieties there are a couple of equivalent ways to introduce the polarization of a algebraic variety. One way is to choose a line bundle $\mathcal{L}$ which satisfies certain ide …
0 votes
1 answer
278 views

Question about Correspondences from Mumford’s Complex Projective Varieties

I study David Mumford's Algebraic Geometry I - Complex Projective Varieties and have some problems to understand a step in the proof of Lemma 6.7 (b). Firstly, the general setting & preparations arou …

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