# Oka theorems

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Theorems on the classical problems in the theory of functions of several complex variables, first proved by K. Oka between 1930 and 1950 (see [1]).

1) Oka's theorem on the Cousin problems: The first Cousin problem is solvable in any domain of holomorphy in ; the second Cousin problem is solvable in any domain of holomorphy that is homeomorphic to , where all domains , except for, possibly, one, are simply connected.

2) Oka's theorem on the Levi problem: Any pseudo-convex Riemannian domain (cf. Pseudo-convex and pseudo-concave) is a domain of holomorphy.

Originally Oka proved these theorems in dimension ; in the case of arbitrary dimension, the theorems were also proved by other mathematicians.

3) The Oka–Weil theorem: Let be a domain in and let the compact set coincide with its hull with respect to the algebra of all functions holomorphic in (cf. Holomorphic envelope); then for any function holomorphic in a neighbourhood of , and for any , a function can be found such that

This fundamental theorem in the theory of holomorphic approximation is extensively used in complex and functional analysis.

4) Oka's coherence theorem: Let be a sheaf of holomorphic functions on a complex manifold ; then for any natural number , any locally finitely-generated subsheaf of the sheaf ( times) is a coherent analytic sheaf (cf. Coherent sheaf).

This is one of the basic theorems of the so-called Oka–Cartan theory, which is used essentially in proving the Cartan theorems and (cf. Cartan theorem).

#### References

 [1] K. Oka, "Sur les fonctions analytiques de plusieurs variables" , Iwanami Shoten (1961) [2] L. Hörmander, "An introduction to complex analysis in several variables" , North-Holland (1973) [3] R.C. Gunning, H. Rossi, "Analytic functions of several complex variables" , Prentice-Hall (1965)