# Analytic surface (in algebraic geometry)

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A two-dimensional (complex) analytic manifold, i.e. a smooth four-dimensional manifold with a complex structure. While the theory of analytic surfaces forms part of the general theory of complex manifolds, the two-dimensional case is treated separately, since much more is known about analytic surfaces than about -dimensional manifolds if . Moreover, certain facts are specific to the two-dimensional case alone. These results concern the classification of analytic surfaces, which is analogous to that of algebraic surfaces (cf. Algebraic surface) — a fact which largely reduces the theory of analytic surfaces to that of algebraic surfaces. The principal results on the classification of analytic surfaces were obtained by K. Kodaira , , , but his work is based on the results of the classical Italian school of algebraic geometry on the classification of algebraic surfaces.

All analytic surfaces discussed below are assumed to be compact and connected.

## Contents

### Examples.

1) Algebraic surfaces. Let be a set of homogeneous polynomials with complex coefficients. The closed subset of the complex projective space specified by the equations is an analytic surface if it is non-singular, connected and has complex dimension two. This is the basic example of an analytic surface.

2) Complex tori. Let be the two-dimensional vector space over the field of complex numbers (as a vector space over the field of real numbers it is isomorphic to ) and let be a lattice in . The quotient space is an analytic surface. Being a smooth manifold, is diffeomorphic to a four-dimensional torus, but the complex structure on depends on the lattice . Complex tori play an important role in analysis, since meromorphic functions on such tori are meromorphic functions on and are periodic with period lattice . Analytic surfaces of the type are not always algebraic. There also exist lattices such that there are no meromorphic functions at all (except for constants) on the corresponding torus . For specific examples of such tori see .

3) Hopf surfaces. Let and let be a positive number. Consider the action of the group on given by The group acts discretely and without fixed points on , while the quotient space is diffeomorphic to . The quotient space has a natural structure of an analytic surface, and is called a Hopf surface.

## Classification of analytic surfaces.

The principal invariant in the classification of analytic surfaces is the transcendence degree of the field of meromorphic functions on the analytic surface . According to Siegel's theorem, for any compact connected manifold the field is finitely generated, and its transcendence degree is not larger than the complex dimension of . Thus, for an analytic surface , the field contains two independent meromorphic algebraic functions, or one such function or constants only. These possibilities lead to the following theorems.

For any analytic surface to be an algebraic surface it is necessary and sufficient that there exist two algebraically independent meromorphic functions on .

If an analytic surface has a field of meromorphic functions of transcendence degree 1, then is an elliptic surface, i.e. there is a holomorphic mapping onto an algebraic curve , , such that and all fibres of , except for a finite number, are elliptic curves (the singular fibres may only have a very special form, which has been thoroughly studied).

If no meromorphic functions other than constants exist on an analytic surface , and no exceptional curves (cf. Exceptional subvariety) exist on , then the first Betti number of assumes only three values: or 0. If , is a complex torus, and if , has a trivial canonical fibration. These analytic surfaces are called -surfaces. They are all mutually homeomorphic. The case has not been studied in detail, but certain examples of analytic surfaces with are obtained by a generalization of the construction of Hopf surfaces.

Analytic Kähler surfaces are not always algebraic. They are, however, algebraic if the square of their first Chern class is positive. All analytic Kähler surfaces with are deformations of algebraic surfaces.

How to Cite This Entry:
Analytic surface (in algebraic geometry). Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Analytic_surface_(in_algebraic_geometry)&oldid=11330
This article was adapted from an original article by B.B. Venkov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article