Function of several complex variables

The theory of functions of several complex variables is the branch of mathematics dealing with complex-valued functions. The name of the field dealing with the properties of function of several complex variables is called several complex variables, that has become a common name for that whole field of study and Mathematics Subject Classification has, as a top-level heading. The function $f:(z_{1},z_{2},\ldots ,z_{n})\rightarrow f(z_{1},z_{2},\ldots ,z_{n})$ is n-tuples of complex numbers, classically considered on the complex coordinate space $\mathbb {C} ^{n}$.

As in complex analysis of functions of one variable, which is the case n = 1, the functions studied are holomorphic or complex analytic so that, locally, they are power series in the variables z_i. Equivalently, they are locally uniform limits of polynomials; or local solutions to the n-dimensional Cauchy–Riemann equations. For one complex variable, the every domain($D\subset \mathbb {C}$), is the domain of holomorphy of some function, in other words every domain has a function for which it is the domain of holomorphy. For several complex variables, this is not the case; there exist domains ($D\subset \mathbb {C} ^{n},\ n\geq 2$) that are not the domain of holomorphy of any function, and so is not always the domain of holomorphy, so the domain of holomorphy is one of the themes in this field. Patching the local data of meromorphic functions, i.e. the problem of creating a global meromorphic function from zeros and poles, is called the Cousin problem. Also, the interesting phenomena that occur in several complex variables are fundamentally important to the study of compact complex manifolds and projective complex varieties ($\mathbb {CP} ^{n}$) and has a different flavour to complex analytic geometry in $\mathbb {C} ^{n}$ or on Stein manifolds.

Historical perspective

Many examples of such functions were familiar in nineteenth-century mathematics; abelian functions, theta functions, and some hypergeometric series. Naturally also same function of one variable that depends on some complex parameter is a candidate. The theory, however, for many years didn't become a full-fledged area in mathematical analysis, since its characteristic phenomena weren't uncovered. The Weierstrass preparation theorem would now be classed as commutative algebra; it did justify the local picture, ramification, that addresses the generalization of the branch points of Riemann surface theory.

With work of Friedrich Hartogs, and of Kiyoshi Oka in the 1930s, a general theory began to emerge; others working in the area at the time were Heinrich Behnke, Peter Thullen and Karl Stein. Hartogs proved some basic results, such as every isolated singularity is removable, for every holomorphic function

$f:\mathbb {C} ^{n}\to \mathbb {C}$

whenever n > 1. Naturally the analogues of contour integrals will be harder to handle: when n = 2 an integral surrounding a point should be over a three-dimensional manifold (since we are in four real dimensions), while iterating contour (line) integrals over two separate complex variables should come to a double integral over a two-dimensional surface. This means that the residue calculus will have to take a very different character.

After 1945 important work in France, in the seminar of Henri Cartan, and Germany with Hans Grauert and Reinhold Remmert, quickly changed the picture of the theory. A number of issues were clarified, in particular that of analytic continuation. Here a major difference is evident from the one-variable theory: while for every open connected set D in $\mathbb {C}$ we can find a function that will nowhere continue analytically over the boundary, that cannot be said for n > 1. In fact the D of that kind are rather special in nature (especially in complex coordinate spaces $\mathbb {C} ^{n}$ and Stein manifolds, satisfying a condition called pseudoconvexity). The natural domains of definition of functions, continued to the limit, are called Stein manifolds and their nature was to make sheaf cohomology groups vanish, also, the property that the sheaf cohomology group disappears is also found in other high-dimensional complex manifolds, indicating that the Hodge manifold is projective. In fact it was the need to put (in particular) the work of Oka on a clearer basis that led quickly to the consistent use of sheaves for the formulation of the theory (with major repercussions for algebraic geometry, in particular from Grauert's work).

From this point onwards there was a foundational theory, which could be applied to analytic geometry, automorphic forms of several variables, and partial differential equations. The deformation theory of complex structures and complex manifolds was described in general terms by Kunihiko Kodaira and D. C. Spencer. The celebrated paper GAGA of Serre pinned down the crossover point from géometrie analytique to géometrie algébrique.

C. L. Siegel was heard to complain that the new theory of functions of several complex variables had few functions in it, meaning that the special function side of the theory was subordinated to sheaves. The interest for number theory, certainly, is in specific generalizations of modular forms. The classical candidates are the Hilbert modular forms and Siegel modular forms. These days these are associated to algebraic groups (respectively the Weil restriction from a totally real number field of GL(2), and the symplectic group), for which it happens that automorphic representations can be derived from analytic functions. In a sense this doesn't contradict Siegel; the modern theory has its own, different directions.

Subsequent developments included the hyperfunction theory, and the edge-of-the-wedge theorem, both of which had some inspiration from quantum field theory. There are a number of other fields, such as Banach algebra theory, that draw on several complex variables.

The complex coordinate space

The complex coordinate space $\mathbb {C} ^{n}$ is the Cartesian product of n copies of $\mathbb {C}$, and when $\mathbb {C} ^{n}$ is a domain of holomorphy, $\mathbb {C} ^{n}$ can be regarded as a Stein manifold. $\mathbb {C} ^{n}$ is also considered to be a Complex projective varieties, a Kähler manifold, etc. It is also an n-dimensional vector space over the complex numbers, which gives its dimension 2n over $\mathbb {R}$. Hence, as a set and as a topological space, $\mathbb {C} ^{n}$ may be identified to the real coordinate space $\mathbb {R} ^{2n}$ and its topological dimension is thus 2n.

In coordinate-free language, any vector space over complex numbers may be thought of as a real vector space of twice as many dimensions, where a complex structure is specified by a linear operator J (such that J ² = −I) which defines multiplication by the imaginary unit i.

Any such space, as a real space, is oriented. On the complex plane thought of as a Cartesian plane, multiplication by a complex number w = u + iv may be represented by the real matrix

${\begin{pmatrix}u&-v\\v&u\end{pmatrix}},$

with determinant

$u^{2}+v^{2}=|w|^{2}.$

Likewise, if one expresses any finite-dimensional complex linear operator as a real matrix (which will be composed from 2 × 2 blocks of the aforementioned form), then its determinant equals to the square of absolute value of the corresponding complex determinant. It is a non-negative number, which implies that the (real) orientation of the space is never reversed by a complex operator. The same applies to Jacobians of holomorphic functions from $\mathbb {C} ^{n}$ to $\mathbb {C} ^{n}$.

Connected space

Every product of a family of an connected (resp. path-connected) spaces is connected (resp. path-connected).

Compact

From Tychonoff's theorem, the space mapped by the cartesian product consisting of any combination of compact spaces is a compact space.

Holomorphic functions

Definition

A function f defined on a domain $D\subset \mathbb {C} ^{n}$ is called holomorphic if f satisfies the following two conditions.

1. f is continuous on D
2. For each variable $z_{\lambda }$, f is holomorphic, namely,
   ${\frac {\partial f}{\partial {\overline {z}}_{\lambda }}}=0$ (1)

   which is a generalization of the Cauchy–Riemann equations.

Cauchy–Riemann equations

For each index λ let

$z_{\lambda }=x_{\lambda }+iy_{\lambda },\quad f(z_{1},\dots ,z_{n})=u(x_{1},\dots ,x_{n},y_{1},\dots ,y_{n})+iv(x_{1},\dots ,x_{n},y_{1},\dots y_{n}),$

and generalize the usual Cauchy–Riemann equation for one variable for each index λ, then we obtain

${\frac {\partial u}{\partial x_{\lambda }}}={\frac {\partial v}{\partial y_{\lambda }}},\ \ \ \ {\frac {\partial u}{\partial y_{\lambda }}}=-{\frac {\partial v}{\partial x_{\lambda }}}.$  (2)

Let

${\begin{aligned}dz_{\lambda }&=dx_{\lambda }+i\,dy_{\lambda },&d{\bar {z}}_{\lambda }&=dx_{\lambda }-i\,dy_{\lambda }\\{\frac {\partial }{\partial z_{\lambda }}}&={\frac {1}{2}}\left({\frac {\partial }{\partial x_{\lambda }}}-i{\frac {\partial }{\partial y_{\lambda }}}\right),&{\frac {\partial }{\partial {\bar {z}}_{\lambda }}}&={\frac {1}{2}}\left({\frac {\partial }{\partial x_{\lambda }}}+i{\frac {\partial }{\partial y_{\lambda }}}\right)\end{aligned}}$ (Wirtinger derivative)

through

$\operatorname {Re} \left({\frac {\partial f}{\partial {\bar {z}}_{\lambda }}}\right)={\frac {\partial u}{\partial x_{\lambda }}}-{\frac {\partial v}{\partial y_{\lambda }}}=0,\quad \operatorname {Im} \left({\frac {\partial f}{\partial {\bar {z}}_{\lambda }}}\right)={\frac {\partial u}{\partial y_{\lambda }}}+{\frac {\partial v}{\partial x_{\lambda }}}=0$

the above equations (1) and (2) turn to be equivalent.

Cauchy's integral formula

f meets the conditions of being continuous and separately homorphic on domain D. Each disk has a rectifiable curve $\gamma$, $\gamma _{\nu }$ is piecewise smoothness, class ${\mathcal {C}}^{1}$ Jordan closed curve. ($\nu =1,2,\ldots ,n$) Let $D_{\nu }$ be the domain surrounded by each $\gamma _{\nu }$. Cartesian product closure ${\overline {D_{1}\times D_{2}\times \cdots \times D_{n}}}$ is ${\overline {D_{1}\times D_{2}\times \cdots \times D_{n}}}\in D$. Also, take the polydisc ${\overline {\Delta }}$ so that it becomes ${\overline {\Delta }}\subset {D_{1}\times D_{2}\times \cdots \times D_{n}}$. (${\overline {\Delta }}(z,r)=\left\{\zeta =(\zeta _{1},\zeta _{2},\dots ,\zeta _{n})\in \mathbb {C} ^{n};\left|\zeta _{\nu }-z_{\nu }\right|\leq r_{\nu }{\text{ for all }}\nu =1,\dots ,n\right\}$ and let $\{z\}_{\nu =1}^{n}$ be the center of each disk.) Using Cauchy's integral formula of one variable repeatedly,

${\begin{aligned}f(z_{1},\ldots ,z_{n})&={\frac {1}{2\pi i}}\int _{\partial D_{1}}{\frac {f(\zeta _{1},z_{2},\ldots ,z_{n})}{\zeta _{1}-z_{1}}}\,d\zeta _{1}\\[6pt]&={\frac {1}{(2\pi i)^{2}}}\int _{\partial D_{2}}\,d\zeta _{2}\int _{\partial D_{1}}{\frac {f(\zeta _{1},\zeta _{2},z_{3},\ldots ,z_{n})}{(\zeta _{1}-z_{1})(\zeta _{2}-z_{2})}}\,d\zeta _{1}\\[6pt]&={\frac {1}{(2\pi i)^{n}}}\int _{\partial D_{n}}\,d\zeta _{n}\cdots \int _{\partial D_{2}}\,d\zeta _{2}\int _{\partial D_{1}}{\frac {f(\zeta _{1},\zeta _{2},\ldots ,\zeta _{n})}{(\zeta _{1}-z_{1})(\zeta _{2}-z_{2})\cdots (\zeta _{n}-z_{n})}}\,d\zeta _{1}\end{aligned}}$

Because $\partial D$ is a rectifiable Jordanian closed curve and f is continuous, so the order of products and sums can be exchanged so the iterated integral can be calculated as a multiple integral. Therefore,

$f(z_{1},\dots ,z_{n})={\frac {1}{(2\pi i)^{n}}}\int _{\partial D_{1}}\cdots \int _{\partial D_{n}}{\frac {f(\zeta _{1},\dots ,\zeta _{n})}{(\zeta _{1}-z_{1})\cdots (\zeta _{n}-z_{n})}}\,d\zeta _{1}\cdots d\zeta _{n}$ (3)

Cauchy's evaluation formula

Because the order of products and sums is interchangeable, from (3) we get

${\frac {\partial ^{k_{1}+\cdots +k_{n}}f(\zeta _{1},\zeta _{2},\ldots ,\zeta _{n})}{\partial {z_{1}}^{k_{1}}\cdots \partial {z_{n}}^{k_{n}}}}={\frac {k_{1}\cdots k_{n}!}{(2\pi i)^{n}}}\int _{\partial D_{1}}\cdots \int _{\partial D_{n}}{\frac {f(\zeta _{1},\dots ,\zeta _{n})}{(\zeta _{1}-z_{1})^{k_{1}+1}\cdots (\zeta _{n}-z_{n})^{k_{n}+1}}}\,d\zeta _{1}\cdots d\zeta _{n}.$ (4)

f is class ${\mathcal {C}}^{\infty }$-function(Smoothness).

From (4), if f is holomorphic, on polydisc $\left\{\zeta =(\zeta _{1},\zeta _{2},\dots ,\zeta _{n})\in \mathbb {C} ^{n};|\zeta _{\nu }-z_{\nu }|\leq r_{\nu },{\text{ for all }}\nu =1,\dots ,n\right\}$ and $|f|\leq {M}$, the following evaluation equation is obtained.

$\left|{\frac {\partial ^{k_{1}+\cdots +k_{n}}f(\zeta _{1},\zeta _{2},\ldots ,\zeta _{n})}{{\partial z_{1}}^{k_{1}}\cdots \partial {z_{n}}^{k_{n}}}}\right|\leq {\frac {Mk_{1}\cdots k_{n}!}{{r_{1}}^{k_{1}}\cdots {r_{n}}^{k_{n}}}}$

Therefore, Liouville's theorem hold.

Power series expansion of holomorphic functions on polydisc

If function f is holomorphic, on polydisc $\{z=(z_{1},z_{2},\dots ,z_{n})\in \mathbb {C} ^{n};|z_{\nu }-a_{\nu }|<r_{\nu },{\text{ for all }}\nu =1,\dots ,n\}$, from Cauchy's integral formula, we can see that it can be uniquely expanded to the next power series.

${\begin{aligned}&f(z)=\sum _{k_{1},\dots ,k_{n}=0}^{\infty }c_{k_{1},\dots ,k_{n}}(z_{1}-a_{1})^{k_{1}}\cdots (z_{n}-a_{n})^{k_{n}}\ ,\\&c_{k_{1}\cdots k_{n}}={\frac {1}{(2\pi i)^{n}}}\int _{\partial D_{1}}\cdots \int _{\partial D_{n}}{\frac {f(\zeta _{1},\dots ,\zeta _{n})}{(\zeta _{1}-a_{1})^{k_{1}+1}\cdots (\zeta _{n}-a_{n})^{k_{n}+1}}}\,d\zeta _{1}\cdots d\zeta _{n}\end{aligned}}$ (5)

In addition, f that satisfies the following conditions is called an analytic function.

For each point $a=(a_{1},\dots ,a_{n})\in D\subset \mathbb {C} ^{n}$, $f(z)$ is expressed as a power series expansion that is convergent on D :

$f(z)=\sum _{k_{1},\dots ,k_{n}=0}^{\infty }c_{k_{1},\dots ,k_{n}}(z_{1}-a_{1})^{k_{1}}\cdots (z_{n}-a_{n})^{k_{n}}\ ,$

We have already explained that holomorphic functions on polydisc are analytic. Also, from the theorem derived by Weierstrass , we can see that the analytic function on polydisc (convergent power series) is holomorphic.

If a sequence of functions $f_{1},\ldots ,f_{n}$ which converges uniformly on compacta inside a domain D, the limit function f of $f_{v}$ also uniformly on compacta inside a domain D. Also, respective partial derivative of $f_{v}$ also compactly converges on domain D to the corresponding derivative of f.

${\frac {\partial ^{k_{1}+\cdots +k_{n}}f}{\partial {z_{1}}^{k_{1}}\cdots \partial {z_{n}}^{k_{n}}}}=\sum _{v=1}^{\infty }{\frac {\partial ^{k_{1}+\cdots +k_{n}}f_{v}}{\partial {z_{1}}^{k_{1}}\cdots \partial {z_{n}}^{k_{n}}}}$

Radius of convergence of power series

It is possible to define a combination of positive real numbers $\{r_{\nu }\ (\nu =1,\dots ,n)\}$ such that the power series ${\textstyle \sum _{k_{1},\dots ,k_{n}=0}^{\infty }c_{k_{1},\dots ,k_{n}}(z_{1}-a_{1})^{k_{1}}\cdots (z_{n}-a_{n})^{k_{n}}\ }$ converges uniformly at $\left\{z=(z_{1},z_{2},\dots ,z_{n})\in \mathbb {C} ^{n};|z_{\nu }-a_{\nu }|<r_{\nu },{\text{ for all }}\nu =1,\dots ,n\right\}$ and does not converge uniformly at $\left\{z=(z_{1},z_{2},\dots ,z_{n})\in \mathbb {C} ^{n};|z_{\nu }-a_{\nu }|>r_{\nu },{\text{ for all }}\nu =1,\dots ,n\right\}$.

In this way it is possible to have a similar, combination of radius of convergence for a one complex variable. This combination is generally not unique and there are an infinite number of combinations.

Bochner–Martinelli formula

The Cauchy integral formula holds only for polydiscs, and in the domain of several complex variables, polydiscs are only one of many domain, so we introduce Bochner–Martinelli formula.

Suppose that f is a continuously differentiable function on the closure of a domain D on $\mathbb {C} ^{n}$ with piecewise smooth boundary $\partial D$. Then the Bochner–Martinelli formula states that if z is in the domain D then, for $\zeta$, z in $\mathbb {C} ^{n}$ the Bochner–Martinelli kernel $\omega (\zeta ,z)$ is a differential form in $\zeta$ of bidegree $(n,n-1)$ defined by

$\omega (\zeta ,z)={\frac {(n-1)!}{(2\pi i)^{n}}}{\frac {1}{|z-\zeta |^{2n}}}\sum _{1\leq j\leq n}({\overline {\zeta }}_{j}-{\overline {z}}_{j})\,d{\overline {\zeta }}_{1}\land d\zeta _{1}\land \cdots \land d\zeta _{j}\land \cdots \land d{\overline {\zeta }}_{n}\land d\zeta _{n}$

$\displaystyle f(z)=\int _{\partial D}f(\zeta )\omega (\zeta ,z)-\int _{D}{\overline {\partial }}f(\zeta )\land \omega (\zeta ,z).$

In particular if f is holomorphic the second term vanishes, so

$\displaystyle f(z)=\int _{\partial D}f(\zeta )\omega (\zeta ,z).$

Identity theorem

When the function f,g is holomorphic in the concatenated domain D, even for several complex variables, the identity theorem holds on the domain D, because it has a power series expansion the neighbourhood of holomorphic point. Therefore, the maximal principle hold. Also, the inverse function theorem and implicit function theorem hold.

Biholomorphism

From the establishment of the inverse function theorem, the following mapping can be defined.

For the domain U, V of the n-dimensional complex space $\mathbb {C} ^{n}$, the bijective holomorphic function $\phi :V\to U$ and the inverse mapping $\phi ^{-1}:V\to U$ is also holomorphic. At this time, $\phi$ is called a U, V biholomorphism also, we say that U and V are biholomorphically equivalent or that they are biholomorphic.

The Riemann mapping theorem does not hold

When $n>1$, open balls and open polydiscs are not biholomorphically equivalent, that is, there is no biholomorphic mapping between the two. This was proven by Poincaré in 1907 by showing that their automorphism groups have different dimensions as Lie groups.

Analytic continuation

Let U, V be domain on $\mathbb {C} ^{n}$, $f\in {\mathcal {O}}(U)$ and $g\in {\mathcal {O}}(V)$. Assume that $U,V,U\cap V\neq \varnothing$ and $W$ is a connected component of $U\cap V$. If $f|_{W}=g|_{W}$ then f is said to be connected to V, and g is said to be analytic continuation of f. From the identity theorem, if g exists, for each way of choosing w it is unique. Whether or not the definition of this analytic continuation is well-defined should be considered whether the domains U,V and W can be defined well. Several complex variables have restrictions on this domain, and depending on the shape of the domain , all holomorphic functions f belonging to U are connected to V, and there may be not exist function f with $\partial U$ as the natural boundary. In other words, U cannot be defined. There is called the Hartogs's phenomenon. Therefore, researching when domain boundaries become natural boundaries has become one of the main research themes of Several complex variables. Also, in the general dimension, there may be multiple intersections between U and V. That is, f is not connected as a single-valued holomorphic function, but as a multivalued holomorphic function. This means that W is not unique and has different properties in the neighborhood of the branch point than in the case of one variable.

Reinhardt domain

Power series expansion of several complex variables it is possible to define the combination of radius of convergence similar to that of one complex variable, but each variable cannot independently define a unique radius of convergence. The Reinhardt domain is considered in order to investigate the characteristics of the convergence domain of the power series, but when considering the Reinhardt domain, it can be seen that the convergence domain of the power series satisfies the convexity called Logarithmically-convex. There are various convexity for the convergence domain of Several complex variables.

A domain D in the complex coordinate space $\mathbb {C} ^{n}$, $n\geq 1$, with centre at a point $a=(a_{1},\dots ,a_{n})\in \mathbb {C} ^{n}$, with the following property; Together with each point $z^{0}=(z_{1}^{0},\dots ,z_{n}^{0})\in D$, the domain also contains the set

$\left\{z=(z_{1},\dots ,z_{n});\left|z_{\nu }-a_{\nu }\right|=\left|z_{\nu }^{0}-a_{\nu }\right|,\ \nu =1,\dots ,n\right\}.$

A Reinhardt domain D with $a=0$ is invariant under the transformations $\left\{z^{0}\right\}\to \left\{z_{\nu }^{0}e^{i\theta _{\nu }}\right\}$, $0\leq \theta _{\nu }<2\pi$, $\nu =1,\dots ,n$. The Reinhardt domains constitute a subclass of the Hartogs domains and a subclass of the circular domains, which are defined by the following condition; Together with all points of $z^{0}\in D$, the domain contains the set

$\left\{z=(z_{1},\dots ,z_{n});z=a+\left(z^{0}-a\right)e^{i\theta },\ 0\leq \theta <2\pi \right\},$

i.e. all points of the circle with center $a$ and radius ${\textstyle \left|z^{0}-a\right|=\left(\sum _{\nu =1}^{n}\left|z_{\nu }^{0}-a_{\nu }\right|^{2}\right)^{\frac {1}{2}}}$ that lie on the complex line through $a$ and $z^{0}$.

A Reinhardt domain D is called a complete Reinhardt domain if together with all point $z^{0}\in D$ it also contains the polydisc

$\left\{z=(z_{1},\dots ,z_{n});\left|z_{\nu }-a_{\nu }\right|\leq \left|z_{\nu }^{0}-a_{\nu }\right|,\ \nu =1,\dots ,n\right\}.$

A complete Reinhardt domain is star-like with respect to its centre a.　Therefore, the complete Reinhardt domain is simply connected, also when the complete Reinhardt domain is the boundary line, there is a way to prove Cauchy's integral theorem without using the Jordan curve theorem.

Logarithmically-convex

A Reinhardt domain D is called logarithmically convex if the image $\lambda (D^{*})$ of the set

$D^{*}=\{z=(z_{1},\dots ,z_{n})\in D;z_{1},\dots ,z_{n}\neq 0\}$

under the mapping

$\lambda ;z\rightarrow \lambda (z)=(\ln |z_{1}|,\dots ,\ln |z_{n}|)$

is a convex set in the real coordinate space $\mathbb {R} ^{n}$.

Every such domain in $\mathbb {C} ^{n}$ is the interior of the set of points of absolute convergence (i.e. the domain of convergence) of some power series in ${\textstyle \sum _{k_{1},\dots ,k_{n}=0}^{\infty }c_{k_{1},\dots ,k_{n}}(z_{1}-a_{1})^{k_{1}}\cdots (z_{n}-a_{n})^{k_{n}}\ }$, and conversely; The domain of convergence of every power series in $z_{1},\dots ,z_{n}$ is a logarithmically-convex Reinhardt domain with centre $a=0$.

Some results

Hartogs's extension theorem and Hartogs's phenomenon

Look at the example on the Hartogs's phenomenon in terms of the Reinhardt domain.

On the polydisk consisting of two disks $\Delta ^{2}=\{z\in \mathbb {C} ^{2};|z_{1}|<1,|z_{2}|<1\}$ when $0<\varepsilon <1$.

Internal domain of $H_{\varepsilon }=\{z=(z_{1},z_{2})\in \Delta ^{2};|z_{1}|<\varepsilon \ \cup \ 1-\varepsilon <|z_{2}|\}\ (0<\varepsilon <1)$

Hartogs's extension theorem (1906); Let f be a holomorphic function on a set G \ K, where G is a domain on $\mathbb {C} ^{n}$ (n ≥ 2) and K is a compact subset of G. If the complement G \ K is connected, then arbitrary holomorphic function f regardless of how it is chosen can be each extended to a unique holomorphic function on G. It is also called Osgood–Brown theorem is that for holomorphic functions of several complex variables, the singularity is a accumulation point, not an isolated point. This means that the various properties that hold for holomorphic functions of one-variable complex variables do not hold for holomorphic functions of several complex variables. The nature of these singularities is also derived from Weierstrass preparation theorem.

From Hartogs's extension theorem the convergence domain extends from $H_{\varepsilon }$ to $\Delta ^{2}$. Looking at this from the perspective of the Reinhardt domain, $H_{\varepsilon }$ is the Reinhardt domain containing the center z = 0, and the convergence domain of $H_{\varepsilon }$ has been extended to the smallest complete Reinhardt domain $\Delta ^{2}$ containing $H_{\varepsilon }$.

Thullen's classic results

Thullen's classical result says that a 2-dimensional bounded Reinhard domain containing the origin is biholomorphic to one of the following domains provided that the orbit of the origin by the automorphism group has positive dimension:

1. $\{(z,w)\in \mathbb {C} ^{2};~|z|<1,~|w|<1\}$ (polydisc);
2. $\{(z,w)\in \mathbb {C} ^{2};~|z|^{2}+|w|^{2}<1\}$ (unit ball);
3. $\{(z,w)\in \mathbb {C} ^{2};~|z|^{2}+|w|^{\frac {2}{p}}<1\}(p>0,\neq 1)$ (Thullen domain).

Sunada's results

Toshikazu Sunada (1978) established a generalization of Thullen's result:

Two n-dimensional bounded Reinhardt domains $G_{1}$ and $G_{2}$ are mutually biholomorphic if and only if there exists a transformation $\varphi :\mathbb {C} ^{n}\to \mathbb {C} ^{n}$ given by $z_{i}\mapsto r_{i}z_{\sigma (i)}(r_{i}>0)$, $\sigma$ being a permutation of the indices), such that $\varphi (G_{1})=G_{2}$.

Domain of holomorphy

The sets in the definition. Note: On this page, replace $\Omega$ in the figure with D

When moving from the theory of one complex variable to the theory of several complex variables, depending on the range of the domain, it may not be possible to define a holomorphic function such that the boundary of the domain becomes a natural boundary. Considering the domain where the boundaries of the domain are natural boundaries (i.e. domain of holomorphy), the first result in the domain of holomorphy (In the complex coordinate space $\mathbb {C} ^{n}$.) was the holomorphic convexity of H. Cartan and Thullen. Levi's problem shows that the pseudoconvex domain was a domain of holomorphy. (First for $\mathbb {C} ^{2}$, later extended to $\mathbb {C} ^{n}$.) Also Kiyoshi Oka's idéal de domaines indéterminés is interpreted by H. Cartan. In Sheaf cohomology theory, the domain of holomorphy has come to be interpreted as the theory of Stein manifolds.

Definition

When a function f is holomorpic on the domain $D\subset \mathbb {C} ^{n}$ and cannot directly connect to the domain outside D, including the point of the domain boundary $\partial D$, the domain D is called the domain of holomorphy of f and the boundary is called the natural boundary of f. In other words, the domain of holomorphy D is the supremum of the domain where the holomorphic function f is holomorphic, and the domain D, which is holomorphic, cannot be extended any more. For Several complex variables, i.e. domain $D\subset \mathbb {C} ^{n}\ (n\geq 2)$, the boundaries may not be natural boundaries. Hartogs' extension theorem gives an example of a domain where boundaries are not natural boundaries.

Formally, a domain D in the n-dimensional complex coordinate space $\mathbb {C} ^{n}$ is called a domain of holomorphy if there do not exist non-empty domain $U\subset D$ and $V\subset \mathbb {C} ^{n}$, $V\not \subset D$ and $U\subset D\cap V$ such that for every holomorphic function f on D there exists a holomorphic function g on V with $f=g$ on U.

For the $n=1$ case, the every domain ($D\subset \mathbb {C}$) was the 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 reciprocal.

Holomorphically convex hull

The first result on the properties of the domain of holomorphy is the holomorphic convexity of Henri Cartan and Peter Thullen (1932).

The holomorphically convex hull of a given compact set in the n-dimensional complex space $\mathbb {C} ^{n}$ is defined as follows.

Let $G\subset \mathbb {C} ^{n}$ be a domain , or alternatively for a more general definition, let $G$ be an $n$ dimensional complex analytic manifold. Further let ${\mathcal {O}}(G)$ stand for the set of holomorphic functions on G. For a compact set $K\subset G$, the holomorphically convex hull of K is

${\hat {K}}_{G}:=\left\{z\in G;|f(z)|\leq \sup _{w\in K}|f(w)|{\text{ for all }}f\in {\mathcal {O}}(G).\right\}.$

One obtains a narrower concept of polynomially convex hull by taking ${\mathcal {O}}(G)$ instead to be the set of complex-valued polynomial functions on G. The polynomially convex hull contains the holomorphically convex hull.

The domain $G$ is called holomorphically convex if for every compact subset $K,{\hat {K}}_{G}$ is also compact in G. Sometimes this is just abbreviated as holomorph-convex.

When $n=1$, every domain $G$ is holomorphically convex since then ${\hat {K}}_{G}$ is the union of $K$ with the relatively compact components of $G\setminus K\subset G$.

If $f$ satisfies the above holomorphically convexity it has the following properties. The radius $\rho$ of the polydisc $\Delta =\{z=(z_{1},z_{2},\dots ,z_{n})\in {\mathbb {C} }^{n};|z_{\nu }-a_{\nu }|<\rho _{\nu },{\text{ for all }}\nu =1,\dots ,n\}$ satisfies condition $\rho _{\nu }=|K,\partial D|=\inf\{|z_{\nu }-z'_{\nu }|;z_{\nu }\in K,z'_{\nu }\in \partial D\}$ also the compact set satisfies $K\subset D$ and $D\subset \mathbb {C} ^{n}$ is the domain. In the time that ($\mathbb {C} ^{n}$), every holomorphic function on the domain $D$ can be direct analytic continuated up to $\Delta$.

Levi convex (approximate from the inside on the analytic polyhedron domain)

${\textstyle \bigcup _{n=1}^{\infty }S_{n}\subseteq D}$ is union of increasing sequence of analytic compact surfaces with paracompact and Holomorphically convex properties such that ${\textstyle \bigcup _{n=1}^{\infty }S_{n}\rightarrow D,S_{n}\subset S_{n+1}}$. i.e. Approximate from the inside by analytic polyhedron.

Pseudoconvex

Pseudoconvex Hartogs showed that $-\log R$ is subharmonic for the radius of convergence in the Hartogs series $R(z)$ when the Hartogs series is a one-variable $f(z,\omega ),\ \omega =0$. If such a relationship holds in the domain of holomorphy of Several complex variables, it looks like a more manageable condition than a holomorphically convex. The subharmonic function looks like a kind of convex function, so it was named by Levi as a pseudoconvex domain. Pseudoconvex domain are important, as they allow for classification of domains of holomorphy.

Definition of plurisubharmonic function

A function

$f\colon D\to {\mathbb {R} }\cup \{-\infty \},$

with domain $D\subset {\mathbb {C} }^{n}$

is called plurisubharmonic if it is upper semi-continuous, and for every complex line

$\{a+bz;z\in \mathbb {C} \}\subset \mathbb {C} ^{n}$ with $a,b\in \mathbb {C} ^{n}$

the function $z\mapsto f(a+bz)$ is a subharmonic function on the set

$\{z\in \mathbb {C} ;a+bz\in D\}.$

In full generality, the notion can be defined on an arbitrary complex manifold or even a Complex analytic space $X$ as follows. An upper semi-continuous function

$f\colon X\to \mathbb {R} \cup \{-\infty \}$

is said to be plurisubharmonic if and only if for any holomorphic map

$\varphi \colon \Delta \to X$ the function

$f\circ \varphi \colon \Delta \to \mathbb {R} \cup \{-\infty \}$

is subharmonic, where $\Delta \subset \mathbb {C}$ denotes the unit disk.

In one-complex variable, necessary and sufficient condition that the real-valued function $u=u(z)$, that can be second-order differentiable with respect to z of one-variable complex function is subharmonic is $\Delta =4\left({\frac {\partial ^{2}u}{\partial z\partial {\overline {z}}}}\right)\geq 0$. There fore, if $u$ is of class ${\mathcal {C}}^{2}$, then $u$ is plurisubharmonic if and only if the hermitian matrix $H_{u}=(\lambda _{ij}),\lambda _{ij}={\frac {\partial ^{2}u}{\partial z_{i}\partial {\bar {z}}_{j}}}$ is positive semidefinite.

Equivalently, a $C^{2}$-function u is plurisubharmonic if and only if ${\sqrt {-1}}\partial {\bar {\partial }}f$ is a positive (1,1)-form.

Strictly plurisubharmonic function

When the hermitian matrix of u is positive-definite and class ${\mathcal {C}}^{2}$, we call u a strict plurisubharmonic function.

(Weakly) pseudoconvex (p-pseudoconvex)

Weak pseudoconvex is defined as : Let $X\subset {\mathbb {C} }^{n}$ be a domain. One says that X is pseudoconvex if there exists a continuous plurisubharmonic function $\varphi$ on X such that the set $\{z\in X;\varphi (z)\leq \sup x\}$ is a relatively compact subset of X for all real numbers x. i.e. there exists a smooth plurisubharmonic exhaustion function $\psi \in {\text{Psh}}(X)\cap {\mathcal {C}}^{\infty }(X)$. Often, the definition of pseudoconvex is used here and is written as; Let X be a complex n-dimensional manifold. Then is said to be weeak pseudoconvex there exists a smooth plurisubharmonic exhaustion function $\psi \in {\text{Psh}}(X)\cap {\mathcal {C}}^{\infty }(X)$.

Strongly (Strictly) pseudoconvex

Let X be a complex n-dimensional manifold. Strongly pseudoconvex if there exists a smooth strictly plurisubharmonic exhaustion function $\psi \in {\text{Psh}}(X)\cap {\mathcal {C}}^{\infty }(X)$,i.e. $H\psi$ is positive definite at every point. The strongly pseudoconvex domain is the pseudoconvex domain. The strong Levi pseudoconvex domain is simply called strong pseudoconvex and is often called strictly pseudoconvex to make it clear that it has a strictly plurisubharmonic exhaustion function in relation to the fact that it may not have a strictly plurisubharmonic exhaustion function.

(Weakly) Levi(–Krzoska) pseudoconvexity

If ${\mathcal {C}}^{2}$ boundary , it can be shown that D has a defining function; i.e., that there exists $\rho :\mathbb {C} ^{n}\to \mathbb {R}$ which is ${\mathcal {C}}^{2}$ so that $D=\{\rho <0\}$, and $\partial D=\{\rho =0\}$. Now, D is pseudoconvex iff for every $p\in \partial D$ and $w$ in the complex tangent space at p, that is,

$\nabla \rho (p)w=\sum _{i=1}^{n}{\frac {\partial \rho (p)}{\partial z_{j}}}w_{j}=0$, we have

$H(\rho )=\sum _{i,j=1}^{n}{\frac {\partial ^{2}\rho (p)}{\partial z_{i}\,\partial {\bar {z_{j}}}}}w_{i}{\bar {w_{j}}}\geq 0.$

For arbitrary complex manifold, Levi (–Krzoska) pseudoconvexity does not always have an plurisubharmonic exhaustion function, i.e. it does not necessarily have a (p-)pseudoconvex domain.

If D does not have a ${\mathcal {C}}^{2}$ boundary, the following approximation result can be useful.

Proposition 1 If D is pseudoconvex, then there exist bounded, strongly Levi pseudoconvex domains $D_{k}\subset D$ with class ${\mathcal {C}}^{\infty }$-boundary which are relatively compact in D, such that

$D=\bigcup _{k=1}^{\infty }D_{k}.$

This is because once we have a $\varphi$ as in the definition we can actually find a ${\mathcal {C}}^{\infty }$ exhaustion function.

Strongly Levi (–Krzoska) pseudoconvexity (Strongly pseudoconvex)

When the Levi (–Krzoska) form is positive-definite, it is called Strongly Levi (–Krzoska) pseudoconvex or often called simply Strongly pseudoconvex.

Levi total pseudoconvex

If for every boundary point $\rho$ of D, there exists an analytic variety ${\mathcal {B}}$ passing $\rho$ which lies entirely outside D in some neighborhood around $\rho$, except the point $\rho$ itself. Domain D that satisfies these conditions is called Levi total pseudoconvex.

Oka pseudoconvex

Family of Oka's disk

Let n-functions $\varphi :z_{j}=\varphi _{j}(u,t)$ be continuous on $\Delta :|U|\leq 1,0\leq t\leq 1$, holomorphic in $|u|<1$ when the parameter t is fixed in [0, 1], and assume that ${\frac {\partial \varphi _{j}}{\partial u}}$ are not all zero at any point on $\Delta$. Then the set $Q(t):=\{Z_{j}=\varphi _{j}(u,t);|u|\leq 1\}$ is called an analytic disc de-pending on a parameter t, and $B(t):=\{Z_{j}=\varphi _{j}(u,t);|u|=1\}$ is called its shell. If $Q(t)\subset D\ (0<t)$ and $B(0)\subset D$, Q(t) is called Family of Oka's disk.

Definition

When $Q(0)\subset D$ holds on any Family of Oka's disk, D is called Oka pseudoconvex. Oka's proof of Levi's problem was that when the unramified Riemannian domain was a domain of holomorphy (holomorphically convex), it was proved that it was necessary and sufficient that each boundary point of the domain of holomorphy is an Oka pseudoconvex.

Cartan pseudoconvex (Local Levi property)

For every point $x\in \partial D$ there exist a neighbourhood U of x and f holomorphic on $\mathbb {C} ^{n}$ ( i.e. $U\cap D$ be holomorphically convex.) such that f cannot be extended to any neighbourhood of x. Such a property is called local Levi property, and the domain that satisfies this property is called the Cartan pseudoconvex domain. In $\mathbb {C} ^{n}$ the Cartan pseudoconvex domain is itself a pseudoconvex domain and is a domain of holomorphy.

Equivalent conditions (In connection with Levi problem)

For a domain $D\subset \mathbb {C} ^{n}$ the following conditions are equivalent.:

1. D is a domain of holomorphy.
2. D is holomorphically convex.
3. D is Levi convex.
4. D is pseudoconvex.
5. D is Cartan pseudoconvex.

The implications $1\Leftrightarrow 2\Leftrightarrow 3$, $1\Rightarrow 4$, and $4\Rightarrow 5$ are standard results. 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 ${\bar {\partial }}$-problem).

Properties of the domain of holomorphy

• If $D_{1},\dots ,D_{n}$ are domains of holomorphy, then their intersection ${\textstyle D=\bigcap _{\nu =1}^{n}D_{\nu }}$ is also a domain of holomorphy.
• If $D_{1}\subseteq D_{2}\subseteq \cdots$ is an increasing sequence of domains of holomorphy, then their union ${\textstyle D=\bigcup _{n=1}^{\infty }D_{n}}$ is also a domain of holomorphy (see Behnke–Stein theorem).
• If $D_{1}$ and $D_{2}$ are domains of holomorphy, then $D_{1}\times D_{2}$ is a domain of holomorphy.
• The first Cousin problem is always solvable in a domain of holomorphy; this is also true, with additional topological assumptions, for the second Cousin problem.

Sheaf

Coherent sheaf

Definition

The definition of the coherent sheaf is as follows.

A coherent sheaf on a ringed space $(X,{\mathcal {O}}_{X})$ is a sheaf ${\mathcal {F}}$ satisfying the following two properties:

1. ${\mathcal {F}}$ is of finite type over ${\mathcal {O}}_{X}$, that is, every point in $X$ has an open neighborhood $U$ in $X$ such that there is a surjective morphism ${\mathcal {O}}_{X}^{n}|_{U}\to {\mathcal {F}}|_{U}$ for some natural number $n$;
2. for any open set $U\subseteq X$, any natural number $n$, and any morphism $\varphi :{\mathcal {O}}_{X}^{n}|_{U}\to {\mathcal {F}}|_{U}$ of ${\mathcal {O}}_{X}$-modules, the kernel of $\varphi$ is of finite type.

Morphisms between (quasi-)coherent sheaves are the same as morphisms of sheaves of ${\mathcal {O}}_{X}$-modules.

Also, Jean-Pierre Serre (1955) proves that

If in an exact sequence $0\to {\mathcal {F}}_{1}|_{U}\to {\mathcal {F}}_{2}|_{U}\to {\mathcal {F}}_{3}|_{U}\to 0$ of sheaves of ${\mathcal {O}}$-modules two of the three sheaves ${\mathcal {F}}_{j}$ are coherent, then the third is coherent as well.

A quasi-coherent sheaf on a ringed space $(X,{\mathcal {O}}_{X})$ is a sheaf ${\mathcal {F}}$ of ${\mathcal {O}}_{X}$-modules which has a local presentation, that is, every point in $X$ has an open neighborhood $U$ in which there is an exact sequence

${\mathcal {O}}_{X}^{\oplus I}|_{U}\to {\mathcal {O}}_{X}^{\oplus J}|_{U}\to {\mathcal {F}}|_{U}\to 0$

for some (possibly infinite) sets $I$ and $J$.

Oka's coherent theorem for sheaf of holomorphic function germ

Kiyoshi Oka (1950) proved the following

Sheaf of holomorphic function germ ${\mathcal {O}}:={\mathcal {O}}_{\mathbb {C} ^{n}}$ on the complex manifold is the coherent sheaf. Therefore, from the above Serre(1955) theorem, ${\mathcal {O}}^{p}$ is also a coherent sheaf. This theorem is also used to prove Cartan's theorems A and B.

Ideal sheaf

If $Z$ is a closed subscheme of a locally Noetherian scheme $X$, the sheaf ${\mathcal {I}}_{Z/X}$ of all regular functions vanishing on $Z$ is coherent. Likewise, if $Z$ is a closed analytic subspace of a complex analytic space $X$, the ideal sheaf ${\mathcal {I}}_{Z/X}$ is coherent.

Cousin problem

In the case of one variable complex functions, Mittag-Leffler's theorem was able to create a global meromorphic function from a given pole, and Weierstrass factorization theorem was able to create a global meromorphic function from a given zero. The theory of Riemann's surface suggests that in multivariate complex functions, the similar theorem that holds for one-variable complex functions does not hold unless Several restrictions are added to the open Complex manifold. This problem is called the Cousin problem and is formulated in Sheaf cohomology terms. They were introduced in special cases by Pierre Cousin in 1895. It was Kiyoshi Oka who gave the complete answer to this question.

First Cousin problem

Definition without Sheaf cohomology words

Each difference $f_{i}-f_{j}$ is a holomorphic function, where it is defined. It asks for a meromorphic function f on M such that $f-f_{i}$ is holomorphic on U_i; in other words, that f shares the singular behaviour of the given local function.

Definition using Sheaf cohomology words

Let K be the sheaf of meromorphic functions and O the sheaf of holomorphic functions on M. If the next map is surjective, Cousin first problem can be solved.

$H^{0}(M,\mathbf {K} ){\xrightarrow {\phi }}H^{0}(M,\mathbf {K} /\mathbf {O} ).$

By the long exact cohomology sequence,

$H^{0}(M,\mathbf {K} ){\xrightarrow {\phi }}H^{0}(M,\mathbf {K} /\mathbf {O} )\to H^{1}(M,\mathbf {O} )$

is exact, and so the first Cousin problem is always solvable provided that the first cohomology group H¹(M,O) vanishes. In particular, by Cartan's theorem B, the Cousin problem is always solvable if M is a Stein manifold.

Second Cousin problem

Definition without Sheaf cohomology words

Each ratio $f_{i}/f_{j}$ is a non-vanishing holomorphic function, where it is defined. It asks for a meromorphic function f on M such that $f/f_{i}$ is holomorphic and non-vanishing.

Definition using Sheaf cohomology words

let $\mathbf {O} ^{*}$ be the sheaf of holomorphic functions that vanish nowhere, and $\mathbf {K} ^{*}$ the sheaf of meromorphic functions that are not identically zero. These are both then sheaves of abelian groups, and the quotient sheaf $\mathbf {K} ^{*}/\mathbf {O} ^{*}$ is well-defined. If the next map $\phi$ is surjective, then Second Cousin problem can be solved.

$H^{0}(M,\mathbf {K} ^{*}){\xrightarrow {\phi }}H^{0}(M,\mathbf {K} ^{*}/\mathbf {O} ^{*}).$

The long exact sheaf cohomology sequence associated to the quotient is

$H^{0}(M,\mathbf {K} ^{*}){\xrightarrow {\phi }}H^{0}(M,\mathbf {K} ^{*}/\mathbf {O} ^{*})\to H^{1}(M,\mathbf {O} ^{*})$

so the second Cousin problem is solvable in all cases provided that $H^{1}(M,\mathbf {O} ^{*})=0.$

The cohomology group $H^{1}(M,\mathbf {O} ^{*}),$ for the multiplicative structure on $\mathbf {O} ^{*}$ can be compared with the cohomology group $H^{1}(M,\mathbf {O} )$ with its additive structure by taking a logarithm. That is, there is an exact sequence of sheaves

$0\to 2\pi i\mathbb {Z} \to \mathbf {O} {\xrightarrow {\exp }}\mathbf {O} ^{*}\to 0$

where the leftmost sheaf is the locally constant sheaf with fiber $2\pi i\mathbb {Z}$. The obstruction to defining a logarithm at the level of H¹ is in $H^{2}(M,\mathbb {Z} )$, from the long exact cohomology sequence

$H^{1}(M,\mathbf {O} )\to H^{1}(M,\mathbf {O} ^{*})\to 2\pi iH^{2}(M,\mathbb {Z} )\to H^{2}(M,\mathbf {O} ).$

When M is a Stein manifold, the middle arrow is an isomorphism because $H^{q}(M,\mathbf {O} )=0$ for $q>0$ so that a necessary and sufficient condition in that case for the second Cousin problem to be always solvable is that $H^{2}(M,\mathbb {Z} )=0.$

Manifolds with several complex variables

Stein manifold (Non-compact manifold)

Since an open Riemann surface always has a non-constant single-valued holomorphic function and satisfies the second axiom of countability, the Riemann surface was considered for embedding the one-dimensional complex plane into a complex manifolds. In fact, taking one point at infinity on the one-dimensional complex plane $\mathbb {C}$ extended it to the Riemann sphere. The Whitney embedding theorem tells us that every smooth n-dimensional manifold can be embedded as a smooth submanifold of $\mathbb {R} ^{2n}$, whereas it is "rare" for a complex manifold to have a holomorphic embedding into $\mathbb {C} ^{n}$. Consider for example arbitrary compact connected complex manifold X: every holomorphic function on it is constant by Liouville's theorem. Now that we know that for Several complex variables, complex manifolds do not always have holomorphic functions that are not constants, consider the conditions that have holomorphic functions. Now if we had a holomorphic embedding of X into $\mathbb {C} ^{n}$, then the coordinate functions of $\mathbb {C} ^{n}$ would restrict to nonconstant holomorphic functions on X, contradicting compactness, except in the case that X is just a point. Complex manifolds that can be embedded into $\mathbb {C} ^{n}$ are called Stein manifolds. Also Stein manifolds satisfy the second axiom of countability.

A Stein manifold is a complex submanifold of the vector space of n complex dimensions. They were introduced by and named after Karl Stein (1951). A Stein space is similar to a Stein manifold but is allowed to have singularities. Stein spaces are the analogues of affine varieties or affine schemes in algebraic geometry. If the univalent domain on $\mathbb {C} ^{n}$ is connection to a manifold, can be regarded as a complex manifold and satisfies the separation condition described later, the condition for becoming a Stein manifold is to satisfy the holomorphic convexity. Therefore, the Stein manifold is the properties of the domain of definition of the (maximal) analytic continuation of an analytic function.

Definition

Suppose X is a paracompact complex manifolds of complex dimension $n$ and let ${\mathcal {O}}(X)$ denote the ring of holomorphic functions on X. We call X a Stein manifold if the following conditions hold:

• X is holomorphically convex, i.e. for every compact subset $K\subset X$, the so-called holomorphically convex hull,

${\bar {K}}=\left\{z\in X;|f(z)|\leq \sup _{w\in K}|f(w)|\ \forall f\in {\mathcal {O}}(X)\right\},$

is also a compact subset of X.

• X is holomorphically separable, i.e. if $x\neq y$ are two points in X, then there exists $f\in {\mathcal {O}}(X)$ such that $f(x)\neq f(y).$
• The open neighborhood of any point on the manifold has a holomorphic Chart to the ${\mathcal {O}}(X)$.

Non-compact (open) Riemann surfaces are Stein

Let X be a connected, non-compact (open) Riemann surface. A deep theorem (1939) of Heinrich Behnke and Stein (1948) asserts that X is a Stein manifold.

Another result, attributed to Hans Grauert and Helmut Röhrl (1956), states moreover that every holomorphic vector bundle on X is trivial. In particular, every line bundle is trivial, so $H^{1}(X,{\mathcal {O}}_{X}^{*})=0$. The exponential sheaf sequence leads to the following exact sequence:

$H^{1}(X,{\mathcal {O}}_{X})\longrightarrow H^{1}(X,{\mathcal {O}}_{X}^{*})\longrightarrow H^{2}(X,\mathbb {Z} )\longrightarrow H^{2}(X,{\mathcal {O}}_{X})$

Now Cartan's theorem B shows that $H^{1}(X,{\mathcal {O}}_{X})=H^{2}(X,{\mathcal {O}}_{X})=0$, therefore $H^{2}(X,\mathbb {Z} )=0$.

This is related to the solution of the second (multiplicative) Cousin problem.

Levi problem

Cartan extended Levi's problem to Stein manifolds.

If the relative compact open subset $D\subset X$ of the Stein manifold X is a Cartan pseudoconvex, then D is a Stein manifold, and conversely, if D is a Cartan pseudoconvex, then X is a Stein manifold. i.e. Then X is a Stein manifold if and only if D is locally the Stein manifold.

This was proved by Bremermann by embedding it in a sufficiently high dimensional $\mathbb {C} ^{m}$, and reducing it to the result of Oka.

Also, Grauert proved for arbitrary complex manifolds.

If the relative compact subset $D\subset X$ of a arbitrary complex manifold X is a strongly pseudoconvex on X, then X is a holomorphically convex (i.e. Stein manifold). Also, D is itself a Stein manifold.

And Narasimhan extended Levi's problem to Complex analytic space, a generalization of complex manifolds.

A Complex analytic space which admits a continuous strictly plurisubharmonic exhaustion function (i.e.strongly pseudoconvex) is Stein space.

Levi's problem remains unresolved in the following cases;

Suppose that X is a singular Stein space, $D\subset \subset X$ . Suppose that for all $p\in \partial D$ there is an open neighborhood $U(p)$ so that $U\cap D$ is Stein. Is $\partial D$ Stein? This means that Behnke–Stein theorem, which holds for Stein manifolds,, has not found a suitable assumption in Stein space.

K-complete

Grauert introduced the concept of K-complete in the proof of Levi's problem.

Let X is complex manifold, X is K-complete if, to each point $x_{0}\in X$, there exist finitely many holomorphic map $f_{1},\dots ,f_{k}$ of X into $\mathbb {C} ^{p}$,$p=p(x_{0})$, such that $x_{0}$ is an isolated point of the set $A=\{x\in X;f^{-1}f(x_{0})\ (v=1,\dots ,k)\}$. This concept also applies to complex space.

Properties and examples of Stein manifolds

• The standard complex space $\mathbb {C} ^{n}$ is a Stein manifold.
• Every domain of holomorphy in $\mathbb {C} ^{n}$ is a Stein manifold.
• It can be shown quite easily that every closed complex submanifold of a Stein manifold is a Stein manifold, too.
• The embedding theorem for Stein manifolds states the following: Every Stein manifold X of complex dimension n can be embedded into $\mathbb {C} ^{2n+1}$ by a biholomorphic proper map.

These facts imply that a Stein manifold is a closed complex submanifold of complex space, whose complex structure is that of the ambient space (because the embedding is biholomorphic).

• Every Stein manifold of (complex) dimension n has the homotopy type of an n-dimensional CW-Complex.
• In one complex dimension the Stein condition can be simplified: a connected Riemann surface is a Stein manifold if and only if it is not compact. This can be proved using a version of the Runge theorem for Riemann surfaces, due to Behnke and Stein.
• Every Stein manifold X is holomorphically spreadable, i.e. for every point $x\in X$, there are n holomorphic functions defined on all of X which form a local coordinate system when restricted to some open neighborhood of x.
• The first Cousin problem can always be solved on a Stein manifold.
• Being a Stein manifold is equivalent to being a (complex) strongly pseudoconvex manifold. The latter means that it has a strongly pseudoconvex (or plurisubharmonic) exhaustive function, i.e. a smooth real function $\psi$ on X (which can be assumed to be a Morse function) with $i\partial {\bar {\partial }}\psi >0$, such that the subsets $\{z\in X\mid \psi (z)\leq c\}$ are compact in X for every real number c. This is a solution to the so-called Levi problem, named after E. E. Levi (1911). The function $\psi$ invites a generalization of Stein manifold to the idea of a corresponding class of compact complex manifolds with boundary called Stein domain. A Stein domain is the preimage $\{z\mid -\infty \leq \psi (z)\leq c\}$. Some authors call such manifolds therefore strictly pseudoconvex manifolds.
• Related to the previous item, another equivalent and more topological definition in complex dimension 2 is the following: a Stein surface is a complex surface X with a real-valued Morse function f on X such that, away from the critical points of f, the field of complex tangencies to the preimage $X_{c}=f^{-1}(c)$ is a contact structure that induces an orientation on X_c agreeing with the usual orientation as the boundary of $f^{-1}(-\infty ,c).$ That is, $f^{-1}(-\infty ,c)$ is a Stein filling of X_c.

Numerous further characterizations of such manifolds exist, in particular capturing the property of their having "many" holomorphic functions taking values in the complex numbers. See for example Cartan's theorems A and B, relating to sheaf cohomology.

In the GAGA set of analogies, Stein manifolds correspond to affine varieties.

Stein manifolds are in some sense dual to the elliptic manifolds in complex analysis which admit "many" holomorphic functions from the complex numbers into themselves. It is known that a Stein manifold is elliptic if and only if it is fibrant in the sense of so-called "holomorphic homotopy theory".

Complex projective varieties (Compact manifold)

Considered in the case of one-variable complex function, the compact Riemann surface had a non-constant single-valued meromorphic function. The compact one-dimensional complex manifold was the Riemann sphere ${\hat {\mathbb {C} }}\cong \mathbb {CP} ^{1}$. However, for high-dimensional (several complex variables) compact complex manifolds, the existence of meromorphic functions cannot be easily indicated because the singularity is not an isolated point. Consider expanding the closed (compact) Riemann surface to a higher dimension, such as, embedding close Complex submanifold M into the complex projective space $\mathbb {CP} ^{n}$, in particular, Complex projective varieties In high-dimensional complex manifolds, the phenomenon that the sheaf cohomology group disappears occurs, and it is Kodaira vanishing theorem and its generalization Nakano vanishing theorem etc. that gives the condition for this phenomenon to occur. Kodaira embedding theorem gives complex Kähler manifold M, with a Hodge metric large enough dimension N into complex projective space, and also embedding from Chow's theorem into Algebraic manifold. These give an example embeddings in manifolds with meromorphic functions. Similarities in Levi problems on the complex projective space $\mathbb {CP} ^{n}$, have been proved in some patterns, for example by Takeuchi.

See also

• Complex geometry
• CR manifold
• Harmonic maps
• Harmonic morphisms
• Infinite-dimensional holomorphy
• Oka–Weil theorem

References

Textbooks

• Behnke, H.; Thullen, P. (1934). Theorie der Funktionen mehrerer komplexer Veränderlichen. doi:10.1007/978-3-642-99659-7. ISBN 978-3-642-98844-8.
• Salomon Bochner and W. T. Martin Several Complex Variables (1948)
• Forster, Otto (1981). Lectures on Riemann surfaces. Graduate Text in Mathematics. Vol. 81. New-York: Springer Verlag. ISBN 0-387-90617-7.
• Grauert, Hans; Remmert, Reinhold (1979), Theory of Stein spaces, Grundlehren der Mathematischen Wissenschaften, vol. 236, Berlin-New York: Springer-Verlag, ISBN 3-540-90388-7, MR 0580152
• B.V. Shabat, Introduction of complex analysis, 1–2, Moscow (1985) (In Russian)
  • Boris Vladimirovich Shabat, Introduction to Complex Analysis, AMS, 1992
  • Introduction to Complex Analysis Part II. Functions of Several Variables. Translations of Mathematical Monographs. Vol. 110. 1992. doi:10.1090/mmono/110. ISBN 9780821819753.
• V.S. Vladimirov, Methods of the theory of functions of many complex variables, M.I.T. (1966) (Translated from Russian)
• Lars Hörmander (1990) [1966], An Introduction to Complex Analysis in Several Variables (3rd ed.), North Holland, ISBN 978-1-493-30273-4
• Cartan, Henri; Takahashi, Reiji (1992). Théorie élémentaire des fonctions analytiques d'une ou plusieurs variables complexes (in French) (6é. ed., nouv. tir ed.). Paris : Hermann. p. 231. ISBN 9782705652159.
  • Elementary theory of analytic functions of one or several complex variables, Dover 1995 (English translation edition)
• Krantz, Steven G. (1992). Function Theory of Several Complex Variables (Second ed.). AMS Chelsea Publishing. p. 340. doi:10.1090/chel/340. ISBN 978-0-8218-2724-6.
• R. Michael Range, Holomorphic Functions and Integral Representations in Several Complex Variables, Springer 1986, 1998
• Volker Scheidemann, Introduction to complex analysis in several variables, Birkhäuser, 2005, ISBN 3-7643-7490-X
• Noguchi, Junjiro (2016). Analytic Function Theory of Several Variables Elements of Oka's Coherence. p. XVIII, 397. doi:10.1007/978-981-10-0291-5. ISBN 978-981-10-0289-2.

Encyclopedia of Mathematics

• Gonchar, A.A.; Shabat, B.V. (2001) [1994], "Analytic function", Encyclopedia of Mathematics, EMS Press
• Solomentsev, E.D. (2001) [1994], "Power series", Encyclopedia of Mathematics, EMS Press
• Solomentsev, E.D. (2001) [1994], "Biholomorphic mapping", Encyclopedia of Mathematics, EMS Press
• Solomentsev, E.D. (2001) [1994], "Reinhardt domain", Encyclopedia of Mathematics, EMS Press
• Chirka, E.M. (2001) [1994], "Hartogs theorem", Encyclopedia of Mathematics, EMS Press
• Onishchik, A.L. (2001) [1994], "Pseudo-convex and pseudo-concave", Encyclopedia of Mathematics, EMS Press
• Solomentsev, E.D. (2001) [1994], "Plurisubharmonic function", Encyclopedia of Mathematics, EMS Press
• Onishchik, A.L. (2001) [1994], "Coherent sheaf", Encyclopedia of Mathematics, EMS Press
• Chirka, E.M. (2001) [1994], "Oka theorems", Encyclopedia of Mathematics, EMS Press
• Chirka, E.M. (2001) [1994], "Cousin problems", Encyclopedia of Mathematics, EMS Press
• Onishchik, A.L. (2001) [1994], "Stein manifold", Encyclopedia of Mathematics, EMS Press

PlanetMath

• This article incorporates material from the following PlanetMath articles, which are licensed under the Creative Commons Attribution/Share-Alike License: Reinhardt domain, Holomorphically convex, Domain of holomorphy, polydisc, biholomorphically equivalent, Levi pseudoconvex.

Further reading

• Krantz, Steven G. (1987), "What is Several Complex Variables?", The American Mathematical Monthly, 94 (3): 236–256, doi:10.2307/2323391, JSTOR 2323391
• Oka, Kiyoshi; R., Remmert(Ed.) (1984), Collected Papers, Springer-Verlag Berlin Heidelberg, p. XIV, 226, ISBN 978-3-662-43412-3

External links

• Tasty Bits of Several Complex Variables open source book by Jiří Lebl
• Complex Analytic and Differential Geometry (OpenContent book See B2)
• Victor Guillemin. 18.117 Topics in Several Complex Variables. Spring 2005. Massachusetts Institute of Technology: MIT OpenCourseWare, https://ocw.mit.edu. License: Creative Commons BY-NC-SA.