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Domain of holomorphy : ウィキペディア英語版
Domain of holomorphy

In mathematics, in the theory of functions of several complex variables, a domain of holomorphy is a set which is maximal in the sense that there exists a holomorphic function on this set which cannot be extended to a bigger set.
Formally, an open set \Omega in the ''n''-dimensional complex space }^n where V is connected, V \not\subset \Omega and U \subset \Omega \cap V such that for every holomorphic function f on \Omega there exists a holomorphic function g on V with f = g on U
In the n=1 case, every open set is a domain of holomorphy: we can define a holomorphic function with zeros accumulating everywhere on the boundary of the domain, which must then be a natural boundary for a domain of definition of its inverse. For n \geq 2 this is no longer true, as it follows from Hartogs' lemma.
== Equivalent conditions ==
For a domain \Omega the following conditions are equivalent:
# \Omega is a domain of holomorphy
# \Omega is holomorphically convex
# \Omega is pseudoconvex
# \Omega is Levi convex - for every sequence S_ \subseteq \Omega of analytic compact surfaces such that S_ \rightarrow S, \partial S_ \rightarrow \Gamma for some set \Gamma we have S \subseteq \Omega (\partial \Omega cannot be "touched from inside" by a sequence of analytic surfaces)
# \Omega has local Levi property - for every point x \in \partial \Omega there exist a neighbourhood U of x and f holomorphic on U \cap \Omega such that f cannot be extended to any neighbourhood of x
Implications 1 \Leftrightarrow 2, 3 \Leftrightarrow 4, 1 \Rightarrow 4, 3 \Rightarrow 5 are standard results (for 1\Rightarrow 3, see Oka's lemma). The main difficulty lies in proving 5 \Rightarrow 1, i.e. constructing a global holomorphic function which admits no extension from non-extendable functions defined only locally. This is called the Levi problem (after E. E. Levi) and was first solved by Kiyoshi Oka, and then by Lars Hörmander using methods from functional analysis and partial differential equations (a consequence of \bar-problem).

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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