Lecture notes on several complex variables

1 Math 650, Fall 2011. Texas A&M University Lecture notes on several complex variables Harold P. Boas Draft of November 30, 2011. Contents 1 Introduction 1. Power series .. 1. Integral representations .. 2. Partial differential equations .. 2. Geometry .. 3. 2 Power series 4. Domain of convergence .. 4. Characterization of domains of convergence .. 5. Elementary properties of holomorphic functions .. 8. Natural boundaries .. 9. Summary: domains of convergence .. 17. The Hartogs phenomenon .. 17. Separate holomorphicity implies joint holomorphicity .. 19. 3 Convexity 23. Real convexity .. 23. Convexity with respect to a class of functions .. 24. Polynomial convexity .. 25. Linear and rational convexity .. 30. ii 1 Introduction Although the great Karl Weierstrass (1815 1897) studied multi- variable power series already in the nineteenth century, the modern theory of several complex variables dates to the researches of Friedrich (Fritz) Hartogs (1874 1943) in the first decade of the twentieth The so-called Hartogs Phenomenon reveals a dramatic difference between one-dimensional complex analysis and multi-dimensional complex analysis, a fundamental feature that had eluded Weierstrass.

2 Some aspects of the theory of holomorphic ( complex analytic) functions the maximum prin- ciple, for example are essentially the same in all dimensions. The most interesting parts of the theory of several complex variables are the features that differ from the one-dimensional theory. The one-dimensional theory is illuminated by several complementary points of view: power se- ries, integral representations, partial differential equations, and geometry. The multi-dimensional theory reveals striking new phenomena from each of these points of view. This chapter sketches some of the issues that will be treated in detail later on. Power series A one- variable power series converges inside a certain disc and diverges outside the closure of the disc.

3 The convergence region for a two-dimensional power series, however, can have infinitely P1 P1 many different shapes. For instance, the largest open set in which the double series n m PnD0 mD0 w converges is the unit bidisc f . ; w/ W j j < 1 and jwj < 1 g, while the series 1 n n nD0 w converges in the unbounded hyperbolic region where j wj < 1. The theory of one-dimensional power series bifurcates into the theory of entire functions (when the series has infinite radius of convergence ) and the theory of functions on the unit disc (when the series has a finite radius of convergence , which can be normalized to the value 1). In higher dimensions, studying power series already leads to function theory on infinitely many dif- ferent types of domains.

4 A natural question, to be answered later, is to characterize the domains that are convergence domains for multi- variable power series. Exercise 1. Exhibit a two- variable power series whose convergence domain is the unit ball f . ; w/ W j j2 C jwj2 < 1 g. 1. A student of Alfred Pringsheim (1850 1941), Hartogs belonged to the Munich school of mathematicians. Because of their Jewish heritage, both Pringsheim and Hartogs suffered greatly under the Nazi regime in the 1930s. Pringsheim, a wealthy man, managed to buy his way out of Germany into Switzerland, where he died at an advanced age in 1941. The situation for Hartogs, however, grew ever more desperate, and in 1943 he committed suicide rather than face being sent to a death camp.

5 1. 1 Introduction Hartogs discovered that every function holomorphic in a neighborhood of the boundary of the unit bidisc automatically extends to be holomorphic on the interior of the bidisc; a proof can be carried out by considering one- variable Laurent series on slices. Thus, in dramatic contrast to the situation in one variable , there are domains in C2 on which all the holomorphic functions extend to a larger domain. A natural question, to be answered later, is to characterize the domains of holomorphy, that is, the natural domains of existence of holomorphic functions. The discovery of Hartogs shows too that holomorphic functions of several variables never have isolated singularities and never have isolated zeroes, in contrast to the one- variable case.

6 Moreover, zeroes (and singularities) must propagate to infinity or to the boundary of the domain where the function is defined. Exercise 2. Let p. ; w/ be a nonconstant polynomial in two variables . Show that the zero set of p cannot be a compact subset of C2 . Integral representations The one- variable Cauchy integral formula for a holomorphic function f on a domain bounded by a simple closed curve C says that 1 f .w/. Z. f . / D dw for inside C : 2 i C w . A remarkable feature of this formula is that the kernel .w / 1 is both universal (independent of the curve C ) and holomorphic in the free variable . There is no such formula in higher dimensions! There are integral representations with a holomorphic kernel that depends on the domain, and there is a universal integral representation with a kernel that is not holomorphic.

7 There is a huge literature about constructing and analyzing integral representations for various special types of domains. Partial differential equations The one-dimensional Cauchy Riemann equations are a pair of real partial differential equations for a pair of functions (the real and imaginary parts of a holomorphic function). In Cn , there are still two functions, but there are 2n equations. Thus when n > 1, the inhomogeneous Cauchy . Riemann equations form an overdetermined system; hence there is a necessary compatibility condition for solvability of the Cauchy Riemann equations. This feature is a significant differ- ence from the one- variable theory. When the inhomogeneous Cauchy Riemann equations are solvable in C2 (or in higher dimen- sion), there is (as will be shown later) a solution with compact support in the case of compactly supported data.

8 When n D 1, however, it is not always possible to solve the inhomogeneous Cauchy Riemann equations while maintaining compact support. The Hartogs phenomenon can be interpreted as a manifestation of this dimensional difference. 2. 1 Introduction Exercise 3. Show that if u is the real part of a holomorphic function of two complex variables 1. (D x1 Ciy1 ) and 2 (D x2 Ciy2 ), then the function u must satisfy the following real second-order partial differential equations: @2 u @2 u @2 u @2 u @2 u @2 u @2 u @2 u C 2 D 0; C 2 D 0; C D 0; D : @x12 @y1 @x22 @y2 @x1 @x2 @y1 @y2 @x1 @y2 @y1 Thus the real part of a holomorphic function of two variables not only is harmonic in each coordinate but also satisfies additional conditions. Geometry In view of the one- variable Riemann mapping theorem, every bounded simply connected planar domain is biholomorphically equivalent to the unit disc.

9 In higher dimension, there is no such simple topological classification of biholomorphically equivalent domains. Indeed, the unit ball in C2 and the unit bidisc in C2 are holomorphically inequivalent domains. One way to understand intuitively why the situation changes in dimension 2 is to realize that in C2 , there is room for one-dimensional complex analysis to happen in the tangent space to the boundary of a domain. Indeed, the boundary of the bidisc contains pieces of one-dimensional complex affine subspaces, while the boundary of the two-dimensional ball does not contain any such analytic disc. Similarly, the zero set of a (not identically zero) holomorphic function in C2 is a one-dimensional complex variety, while the zero set of a holomorphic function in C1 is a zero-dimensional variety (that is, a discrete set of points).

10 There is a mismatch between the dimension of the domain and the dimension of the range of a multi- variable holomorphic function. One might expect an equidimensional holomorphic mapping to be analogous to a one- variable holomorphic function. Here too there are surprises. For instance, there exists a biholomorphic mapping from all of C2 onto a proper subset of C2. whose complement has interior points. Such a mapping is called a Fatou Bieberbach 2. The name honors the French mathematician Pierre Fatou (1878 1929) and the German mathematician Ludwig Bieberbach (1886 1982). 3. 2 Power series Examples in the introduction show that the domain of convergence of a multi- variable power series can have various shapes; in particular, the domain need not be a convex set.