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Remmert-Stein theorem (Theorem)

For a complex analytic subvariety $ V$ and $ p \in V$ a regular point, let $ \dim_p V$ denote the complex dimension of $ V$ near the point $ p.$

Theorem 1 (Remmert-Stein)   Let $ U \subset {\mathbb{C}}^n$ be a domain and let $ S$ be a complex analytic subvariety of $ U$ of dimension $ m < n.$ Let $ V$ be a complex analytic subvariety of $ U \backslash S$ such that $ \dim_p V > m$ for all regular points $ p \in V.$ Then the closure of $ V$ in $ U$ is an analytic variety in $ U.$

The condition that $ \dim_p V > m$ for all regular $ p$ is the same as saying that all the irreducible components of $ V$ are of dimension strictly greater than $ m.$ To show that the restriction on the dimension of $ S$ is “sharp,” consider the following example where the dimension of $ V$ equals the dimension of $ S$. Let $ (z,w) \in {\mathbb{C}}^2$ be our coordinates and let $ V$ be defined by $ w = e^{1/z}$ in $ {\mathbb{C}}^2 \setminus S,$ where $ S$ is defined by $ z = 0.$ The closure of $ V$ in $ {\mathbb{C}}^2$ cannot possibly be analytic. To see this look for example at $ W = \overline{V} \cap \{ w = 1 \}.$ If $ \overline{V}$ is analytic then $ W$ ought to be a zero dimensional complex analytic set and thus a set of isolated points, but it has a limit point $ (0,1)$ by Picard's theorem.

Finally note that there are various generalizations of this theorem where the set $ S$ need not be a variety, as long as it is of small enough dimension. Alternatively, if $ V$ is of finite volume, we can weaken the restrictions on $ S$ even further.

Bibliography

1
Klaus Fritzsche, Hans Grauert. From Holomorphic Functions to Complex Manifolds, Springer-Verlag, New York, New York, 2002.
2
Hassler Whitney. Complex Analytic Varieties. Addison-Wesley, Philippines, 1972.



"Remmert-Stein theorem" is owned by jirka. [ full author list (2) ]
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See Also: Chow's theorem

Other names:  Remmert-Stein extension theorem
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Cross-references: even, volume, finite, variety, Picard's theorem, limit point, isolated points, zero dimensional, analytic, coordinates, restriction, strictly, irreducible components, regular, analytic variety, closure, point, near, dimension, complex, regular point, complex analytic subvariety
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This is version 6 of Remmert-Stein theorem, born on 2005-02-22, modified 2008-02-04.
Object id is 6805, canonical name is RemmertSteinTheorem.
Accessed 1830 times total.

Classification:
AMS MSC32A60 (Several complex variables and analytic spaces :: Holomorphic functions of several complex variables :: Zero sets of holomorphic functions)
 32C25 (Several complex variables and analytic spaces :: Analytic spaces :: Analytic subsets and submanifolds)

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