# Hartogs's extension theorem

Hartogs's extension theorem "Hartogs' lemma" redirects here. For the lemma on infinite ordinals, see Hartogs number.

In the theory of functions of several complex variables, Hartogs's extension theorem is a statement about the singularities of holomorphic functions of several variables. Informally, it states that the support of the singularities of such functions cannot be compact, therefore the singular set of a function of several complex variables must (loosely speaking) 'go off to infinity' in some direction. More precisely, it shows that an isolated singularity is always a removable singularity for any analytic function of n > 1 complex variables. A first version of this theorem was proved by Friedrich Hartogs,[1] and as such it is known also as Hartogs's lemma and Hartogs's principle: in earlier Soviet literature,[2] it is also called Osgood–Brown theorem, acknowledging later work by Arthur Barton Brown and William Fogg Osgood.[3] This property of holomorphic functions of several variables is also called Hartogs's phenomenon: however, the locution "Hartogs's phenomenon" is also used to identify the property of solutions of systems of partial differential or convolution equations satisfying Hartogs type theorems.[4] Contents 1 Historical note 2 Hartogs's phenomenon 3 Formal statement and proof 4 Counterexamples in dimension one 5 Notes 6 References 6.1 Historical references 6.2 Scientific references 7 External links Historical note The original proof was given by Friedrich Hartogs in 1906, using Cauchy's integral formula for functions of several complex variables.[1] Today, usual proofs rely on either the Bochner–Martinelli–Koppelman formula or the solution of the inhomogeneous Cauchy–Riemann equations with compact support. The latter approach is due to Leon Ehrenpreis who initiated it in the paper (Ehrenpreis 1961). Yet another very simple proof of this result was given by Gaetano Fichera in the paper (Fichera 1957), by using his solution of the Dirichlet problem for holomorphic functions of several variables and the related concept of CR-function:[5] later he extended the theorem to a certain class of partial differential operators in the paper (Fichera 1983), and his ideas were later further explored by Giuliano Bratti.[6] Also the Japanese school of the theory of partial differential operators worked much on this topic, with notable contributions by Akira Kaneko.[7] Their approach is to use Ehrenpreis's fundamental principle.

Hartogs's phenomenon For example, in two variables, consider the interior domain {displaystyle H_{varepsilon }={z=(z_{1},z_{2})in Delta ^{2}:|z_{1}|

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