Complex geometry is the study of complex manifolds and complex algebraic varieties. It is a part of both differential geometry and algebraic geometry.

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3
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35 views

About rational curves on elliptic K3 surfaces

I am trying to read this paper http://arxiv.org/pdf/math/9902092.pdf. I have some trouble at the end in Corollary 3.27, and also Proposition 3.24. 0)Morally speaking my main trouble is that i don't ...
2
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3answers
44 views

Holomorphic curve with unit norm

Is there an open set $U \subset \mathbb{C}$ and a holomorphic function $\gamma: U \to \mathbb{C}^{2}$ such that $\forall z \in U: \|\gamma(z) \| =1$? if the answer is yes, can the method of unit ...
6
votes
1answer
100 views

‘Integral’ of a Weierstrass $ \wp $-function.

I'm revising for my finals and I've seen a question which asks: Is there a meromorphic function $f: \mathbb{C}/\Lambda \to \mathbb{P}^1$ such that $f' = \wp$? There is a hint which says consider the ...
1
vote
1answer
71 views

What exactly is the Kahler class of a torus?

Is there an intuitional way to understand what the Kahler class of $T^2$ actually is? It would be extremely useful to me if you could provide me some intuition behind it!
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88 views

What's Griffiths' ampleness in modern language?

When reading Griffiths' paper "Hermitian differential geometry, Chern classes and positive vector bundles", I found his definition of ampleness: Let $X$ be a compact complex manifold, $E\to X$ a ...
2
votes
1answer
34 views

Description of $(T\Bbb{CP}^1)^\perp$

Is there a nice "concrete" description (i.e., coordinates) of the normal bundle of $\Bbb{CP}^1$ when is considered as a submanifold of $\Bbb{CP}^n$? Or, at least, $\Bbb{CP}^2$?
5
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2answers
183 views

References to understand $K3$ surface as a double cover of $\mathbb{P}^2$ ramified along a sextic

My goal is to understand that $2:1$ cover of $\mathbb{P}^2(\mathbb{C})$ ramified along a sextic is a $K3$ surface. My main problem is in understanding the theory of ramified covering of ...
1
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0answers
24 views

Dévissage for complex manifolds

In algebraic geometry one has the following result: Let $X$ be a noetherian scheme and $\mathcal{F}$ a coherent sheaf with support $Z \neq X$. Then $\mathcal{F}$ has a finite filtration $\mathcal{F} ...
1
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1answer
64 views

What is the meaning of this statement about complex structure?

I get confused when in papers it is said that: "Something is holomorphic (Complex, symplectic, etc ...) in some Complex structure" What is the meaning of this in general? For example in a paper ...
1
vote
1answer
105 views

Complex hypersurface globally defined

Let $A$ be a pure one-codimensional analytic subset of a domain $D \subset \mathbb{C}^n$. Is it true that $A$ is defined by one single holomorphic equation $f(z)=0$ if $D$ is bounded and pseudo-convex ...
0
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0answers
38 views

Deformation theory in a paper of Bogomolov and Tschinkel

I am trying to read this paper http://www.math.nyu.edu/~tschinke/papers/yuri/00ajm/ajm.pdf, by Bogomolov and Tschinkel. I had 0 preparation in the theory of deformation of complex structures, but in ...
4
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2answers
107 views

Does a complex manifold always admit an acyclic cover for the sheaf of holomorphic functions?

Let $\mathcal{F}$ be a sheaf on a topological space $X$. An open cover $\mathcal{U} = \{U_i\}_{i\in I}$ of $X$ is called acyclic for $\mathcal{F}$ if for all $i_0, \dots, i_p \in I$, ...
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vote
0answers
32 views

When the associated bundle to a $U(1)$-bundle is an holomorphic line bundle?

Let $P$ be a principal $U(1)$ bundle over a complex manifold $M$, and let $\rho\colon U(1)\to Aut(\mathbb{C})$ be the representation of $U(1)$ on $\mathbb{C}$ given by the standard multiplication. My ...
3
votes
0answers
76 views

Correspondence between line bundles and $U(1)$-bundles: a mistake from the physicists?

I am reading a paper written by physicists and they say the following: Let $(L,h,\nabla)$ be an holomorphic line bundle equipped with a Hermitian metric $h$ and Chern connection $\nabla$. If ...
3
votes
1answer
155 views

Classification of line bundles by Griffiths and Harris

I am reading pages $132$ and $133$ of Principles of Algebraic Geometry by Griffiths and Harris. They consider a holomorphic line bundle $L \to M$ over a manifold $M$ and an open cover $\left\{ ...
3
votes
1answer
116 views

Ricci curvature of the Grassmannian?

Let $G(k, \mathbb{C}^n)$ be the Grassmannian of $k-$dimensional complex linear subspaces of $\mathbb{C}^n.$ We know that the Grassmannian can be embedded to the projective space ...
2
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0answers
90 views

Understanding the isomorphism of Picard group with the first cohomology group

I am learning the subject for the first time, and the material has not yet settled inside me. I would like to get some intuitive understanding of the following: Let $X$ be a complex manifold. The ...
8
votes
2answers
115 views

How can hypersurfaces “know” the degree of their defining polynomials?

I'm currently trying to learn some complex and projective geometry. There is one issue bugging me again and again, from different perspectives, and I just can't get my head around it. One incarnation ...
0
votes
0answers
51 views

Geometrical relationship between the points in the Argand diagram

Explain the geometrical relationship between the points in the Argand diagram represented by the complex numbers $z$ and $a + (z - a)e^{i\theta}$ (1) Write down the necessary and sufficient ...
1
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0answers
54 views

Commutative diagram of cohomology (to show Albanese variety is a torus)

Suppose $X$ is a compact Kahler manifold of complex dimension $n$, define $H_1(X,\mathbb{Z})\to H^0(X,\Omega_X^1)^*$ by $[\alpha]\to \int_\alpha\cdot-$. We want to show the image of ...
2
votes
1answer
40 views

Does acyclic resolution induces morphism of cohomologies?

Consider sheaves $F,G$ of abelian groups on a topological space $X$. Fix some $f^0\colon F\to G$. Given chain map between injective resolutions $0\to F\to I^*$, $0\to G\to J^*$, denoted by $f^0\colon ...
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0answers
54 views

Can you have a “real” decomposition of differential forms on a symplectic manifold?

With a choice of (almost) complex structure on a symplectic (Kahler) manifold, has anyone thought of how or what it means to globally define "real" versions of the Dolbeault operators? i.e. Something ...
0
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0answers
29 views

Why the origin of this complex is moving away from the origin (0,0)?

Why does the origin of the complex line z is moving away from the origin? $$Let\;z=x+i\cdot y \\\;z-1\;=\;\;(x-1)+i\cdot y\;$$ Following that I would say that the coordinates of the origin of z are ...
1
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1answer
51 views

Kähler–Einstein condition

Let $(M,\omega)$ be a Kähler manifold, and $g$ the Riemannian metric such that $\omega(X,JY)=g(X,Y)$. If there is a function $f$ such that $\operatorname{Ric}=f\omega$ does this mean that ...
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0answers
49 views

How to see whether a tori is an abelian variety or not?

Given an explicit lattice $\Lambda \cong \mathbb{Z}^{2n}$ in $\mathbb{C}^n$, how can one check whether the complex torus $\mathbb{C}^n/\Lambda$ is a projective or not?
4
votes
0answers
122 views

Analytic variety is a countable union of complex manifolds

In an article on real analytic manifolds I came across the following remark: Let $W$ be a purely $k$-dimensional analytic subvariety of a domain in $\mathbb{C}^n$ and let $S$ be its singular locus. ...
0
votes
1answer
31 views

Based on the Andreotti-Frankel theorem, what is the CW complex homotopy equivalent to $x^2 + y^2 - 1$?

I am referring to this theorem: http://en.wikipedia.org/wiki/Andreotti%E2%80%93Frankel_theorem I have no idea how to begin thinking about this.
4
votes
1answer
62 views

What are the irreducible curves on the blow up of $\mathbb{P}^{2}$?

On the blow-up of $2$-dimensional complex projective space, $\mathbb{P}^2$, I know that $$Pic(\mathbb{\tilde{P}^2})=\mathbb{Z}[\tilde{H}]+\mathbb{Z}[E]$$ where $\tilde{H}$ is the blow-up of the ...
3
votes
0answers
36 views

Complete intersections with respect to different sections

Let us consider the Whitney sum $E$ of holomorphic line bundles $L_1,\dots, L_k$ on a smooth (projective) variety $X$. For a generic global section $s$ of $E$, the zero locus $Z_s := s^{-1}(0_E)$ ...
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335 views

Higher dimensional analogue for Riemann Hurwitz formula

There are few questions like here and here already asked about this. But I don't have the background to understand the answers there. I am just beginning to learn classical algebraic geometry, and ...
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0answers
46 views

Fubini-Study norm of homogeneus polynomials

Let us consider the complex projective line $\mathbb{P}^1(\mathbb{C})$, by direct exam of the degrees we have the isomorphism: $$ T \mathbb{P}^1(\mathbb{C}) \simeq ...
2
votes
1answer
65 views

How local is the exponent in the definition of a function with analytic/algebraic singularities?

In Demailly's Analytic Methods in Algebraic Geometry (available on his web page), the definition of a (plurisubharmonic) "function with analytic singularities" is a (plurisubharmonic) function $ u: ...
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1answer
42 views

Is the dimension of a linear system $|D|$ always finite?

Let $M$ be a compact complex manifold and $D=\sum a_i V_i$ a divisor on $M.$ If $|D|$ is the set of effective divisors linearly equivalent to $D,$ we know that $$|D|\cong \mathbb{P}( H^0 (M, {\cal ...
3
votes
1answer
69 views

Global sections of holomorphic vector bundles

Let $X$ be a complex manifold, and $\mathbb{L}\rightarrow X$ a holomorphic line bundle over $X.$ Can we always find global sections of $\mathbb{L}$? (other from the one that's identically zero) On a ...
3
votes
1answer
115 views

Grassmannian a Fano manifold?

I am interested in answering the following: Is it true that the Grassmannian $G(k,\mathbb{C}^n)$ is a Fano manifold? How can I see if its anticanonical bundle is ample?
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vote
2answers
35 views

Analytic hypersurface as union of irreducibles

Let $X$ be a complex manifold. Then any analytic subvariety $V$ of codimension 1 (that is, any analytic hypersurface) can be expressed uniquely as the union of irreducible analytic hypersurfaces ...
0
votes
1answer
37 views

What is the linear series $|mL|$?

I am studying complex geometry and I am trying to find out what is the definition of the linear series $|mL|,$ where $L$ be a line bundle over a compact Kahler manifold $X^n.$ In particular, I know ...
0
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0answers
41 views

Fundamental cycle $[Y]\in H_{2n-2}(X_{\mathbb{R}},\mathbb{Z})$ of irreducible analytic hypersurface on a complex manifold X.

I am studying about complex manifolds and I am trying to understand the following statement. Let $Y\subset X$ be an irreducible analytic hypersurface, where $X$ is an n-dimensional complex compact ...
5
votes
1answer
83 views

What is the canonical bundle of a smooth divisor?

I am currently trying to learn some complex geometry, using mainly the book by Huybrechts. There is one thing confusing me, however: For example on Wikipedia people are talking about the canonical ...
0
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0answers
50 views

Is the determinant bundle the pullback of the $\mathcal O(1)$ on $\mathbb P^n$ under the Plucker embedding?

Let $V$ be a $n$-dimensional complex vector space and consider the Grassmannian of complex $k$-planes $Gr(k,V)$. The Plucker embedding is an embedding $p:Gr(k,V) \to \mathbb P^M$ where $M = ...
2
votes
0answers
81 views

Smoothness and algebraicity

My question is the following : given a complex algebraic affine variety $M \subset \mathbb{C}^n$, let us assume that it has moreover a smooth differential structure as a submaifold of ...
4
votes
1answer
100 views

How do we wedge the complex differentials $\mathrm{d}z^i$ and $\mathrm{d}\bar z^{\bar j}$?

By the standard definition of the wedge product as an alternated tensor product, I would think we have $$\tag{1}\mathrm{d}z^i\wedge\mathrm{d}\bar z^{\bar j}=\mathrm{d}z^i\otimes\mathrm{d}\bar z^{\bar ...
7
votes
2answers
114 views

What are some good sources to learn about real analytic manifolds?

Asking this question as someone with a graduate student level understanding of smooth differential/Riemannian geometry (May be a bit more that Riemannian Geometry by Do Carmo). I am trying to upgrade ...
0
votes
1answer
44 views

Naive question about almost complex structures and hermitian metrics

I'm sorry if I have another naive question, but I want to understand correctly almost complex structures and hermitian metrics. Let's have $M$ complex manifold with almost complex structure $J$ and ...
2
votes
1answer
99 views

Why is the study of cones important in algebraic geometry?

It was quite difficult writing this question...I hope you can understand the goal of that. I want to ask it in order to clarify my ideas. I'm studying Hyperkahler geometry (principally on works by ...
0
votes
1answer
30 views

is that function must be constant under the following conditions

I'm talking about complex function $f$ is analytic function on a region $D$ that include the point $z=0$. for every $n\in N$ such $\frac{1}{n}$ is in $D$ , the function follows this condition ...
3
votes
1answer
126 views

Interpretation of the Weierstrass Preparation Theorem

I am reading Griffiths and Harris' Principles of Algebraic Geometry but I am having trouble making sense of a statement following the Weierstrass Preparation Theorem (p.9 in my edition). The ...
0
votes
0answers
20 views

Newlander-Nerenberg theorem in Voisin's book

I am reading the proof of Newlander-Nerenberg's theorem in the real analytic case, and there are some parts I don't understand, can someone help me please? 1) In the beginning, she said: " since ...
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0answers
43 views

Examples of complex manifolds satisfying the Kodaira-Spencer condition for unobstructed deformations

The theorem of existence proved by Kodaira and Spencer shows that a complex manifold $M$ for which the second cohomology group $H^2(M, \Theta T^{1,0} M)$ is trivial, $\Theta T^{1,0} M$ being the sheaf ...
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48 views

the laplacian in real coordinates and complex coordinates

In the notes by Szekely it says on page 35 that since $\nabla_k\frac{\partial}{\partial \overline z_l}=0$ it follows in holomorphic coordinates that the Laplacian is $\Delta f=g^{j\overline ...