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Complex analysis, holomorphic functions, automorphic group actions and forms, pseudoconvexity, complex geometry, analytic spaces, analytic sheaves.

29 votes

Pathology in Complex Analysis

I would say that Mandelbrot set (like similar fractal objects coming from complex dynamics) can be seen as a "pathological" object, at least from the point of view of regularity (the boundary is nowhe …
10 votes
Accepted

Is polynomial convexity a topological invariant?

The answer is no. In fact, Kallin has shown in [Kal64] that the union of three disjoint closed balls is polynomially convex, but the union of three disjoint closed polydisks needs not to be polynomi …
Francesco Polizzi's user avatar
13 votes

What is the Krull dimension of the ring of holomorphic functions on a complex manifold?

Are you also looking for holomorphic manifolds with $\dim \mathcal O=\infty$? In that case, in the paper by Sasane On the Krull Dimension of Rings of Transfer Functions [Acta Applicandae Mathematicae …
Francesco Polizzi's user avatar
2 votes
Accepted

Necessary condition for a branch point

Your question is not very clear. However, I guess you are asking for the branch points of the cover of $\mathbb{C}$ defined by $f(z, \alpha)=0$. In this case, let us assume for the sake of symplicity …
Francesco Polizzi's user avatar
7 votes
Accepted

Intersection multiplicity in the non-algebraic case

If we work in the category of holomorphic functions, then we can give the following definition, that generalises to the complex-analytic setting the classical intersection multiplicity used in algebra …
Francesco Polizzi's user avatar
10 votes
Accepted

Is $\mathbb{CP}^2$ with a line collapsed a complex analytic space?

The answer is no, because of the following general result. Theorem (Grauert's contractibility criterion). Let $X$ be a smooth complex surface and let $E \subset X$ be a connected curve in $X$, wit …
Francesco Polizzi's user avatar
6 votes
Accepted

Zariski's main theorem in the complex analytic category

One reference is Proposition 14.7 in Remmert's paper Local Theory of complex analytic spaces, Several complex variable VII, Encyclopaedia of Math. Sci. vol 74. For the reader's convenience I will rest …
Francesco Polizzi's user avatar
8 votes
Accepted

Equivalence of Branched Coverings

The answer is yes: the equivalence class of the covering is detected by the monodromy representation of the fundamental group of the base minus the branch locus, up to conjugacy. More precisely, let …
Francesco Polizzi's user avatar
16 votes
Accepted

Fundamental Groups of compact Complex manifolds?

Every finitely presented group is the fundamental group of a compact complex manifold of dimension $3$. This is proven in the book by Amoros, Burger, Corlette, Kotschick and Toledo Fundamental group …
Francesco Polizzi's user avatar
11 votes

Riemann surfaces with an atlas all of whose open sets are biholomorphic to $\mathbb{C}$?

Compact Riemann surfaces of genus at least $2$ are uniformized by the unit disk, hence they do not admit any non-constant holomorphic map from $\mathbb{C}$.
Francesco Polizzi's user avatar
15 votes
Accepted

Algebraic vs analytic normality

Over $\mathbf{C}$, algebraic normalization and analytic normalization are equivalent concepts. See N. Kuhlmann: Die Normalisierung komplexer Räume, Math. Ann. 144 (1961), 110-125, ZBL0096.27801. Qu …
Francesco Polizzi's user avatar
12 votes

An analytic proof of the De Franchis theorem

Let me give a proof using the deformation theory of holomorphic maps developed by Horikawa [Journal Math. Soc. Japan 25]. This can be seen as a purely analytic proof in the spirit of Kodaira's "defor …
Francesco Polizzi's user avatar
2 votes

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

Some standard examples are Hopf surfaces, obtained as quotients of the form $X=\mathbb{C}^2 - \{0\} /G$, where $G$ is the subgroup generated by the homothety $(z_1, z_2) \to (\alpha_1 z_1, \alpha …
Francesco Polizzi's user avatar
26 votes

Which almost complex manifolds admit a complex structure?

There are actually counterexamples in real dimension $4$. The first examples of compact almost complex $4$-manifolds admitting no complex structure were produced by Van de Ven in his paper "On the Ch …
Francesco Polizzi's user avatar
3 votes

Subset of a complex manifold whose intersection with every holomorphic curve is analytic

I think that the answer is no. Take a complex torus $X$ without any holomorphic curve. If the result you are asking for were true, it would imply (in the empty sense) that every subset $A \subset X$ i …
Francesco Polizzi's user avatar

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