# .

# Hopf surface

In complex geometry, a **Hopf surface** is a compact complex surface obtained as a quotient of the complex vector space (with zero deleted) **C**^{2} \ 0 by a free action of a discrete group. If this group is the integers the Hopf surface is called **primary**, otherwise it is called **secondary**. (Some authors use the term "Hopf surface" to mean "primary Hopf surface".) The first example was found by Hopf (1948), with the discrete group isomorphic to the integers, with a generator acting on **C**^{2} by multiplication by 2; this was the first example of a compact complex surface with no Kähler metric.

Higher-dimensional analogues of Hopf surfaces are called Hopf manifolds.

Invariants

Hopf surfaces are surfaces of class VII and in particular all have Kodaira dimension −∞ and all their plurigenera vanish. The geometric genus is 0. The fundamental group has a normal central infinite cyclic subgroup of finite index. The Hodge diamond is

1 | ||||

0 | 1 | |||

0 | 0 | 0 | ||

1 | 0 | |||

1 |

In particular the first Betti number is 1 and the second Betti number is 0. Conversely Kodaira (1968) showed that that a compact complex surface with vanishing the second Betti number and whose fundamental group contains an infinite cyclic subgroup of finite index is a Hopf surface.

Primary Hopf surfaces

In the course of classification of compact complex surfaces, Kodaira classified the primary Hopf surfaces.

A primary Hopf surface is obtained as

\( H=\bigg({\Bbb C}^2\backslash 0\bigg)/\Gamma, \)

where \( \Gamma \) is a group generated by a polynomial contraction \( \gamma \) . Kodaira has found a normal form for \( \gamma \) . In appropriate coordinates, \gamma can be written as

\( (x, y) \mapsto (\alpha x +\lambda y^n, \beta y) \)

where \( \alpha, \beta\in {\Bbb C} \) are complex numbers satisfying \( 0<|\alpha|\leq |\beta| <1 \) , and either \( \;\lambda=0 \) or \( \;\alpha=\beta^n. \)

These surfaces contain an elliptic curve (the image of the *x*-axis) and if λ=0 the image of the *y*-axis is a second elliptic curve. When λ=0, the Hopf surface is an elliptic fiber space over the projective line if α^{m} =β^{n} for some positive integers *m* and *n*, with the map to the projective line given by *x*^{n}*y*^{−m}, and otherwise the only curves are the two images of the axes.

The Picard group of any primary Hopf surface is isomorphic to the non-zero complex numbers **C**^{*}.

Kodaira (1966b) has proven that a complex surface is diffeomorphic to **S**^{3}×**S**^{1} if and only if it is a primary Hopf surface.

Secondary Hopf surfaces

Any secondary Hopf surface has a finite unramified cover that is a primary Hopf surface. Equivalently, its fundamental group has a subgroup of finite index in its center that is isomorphic to the integers. Kato (1975) classified them by finding the finite groups acting without fixed points on primary Hopf surfaces.

Many examples of secondary Hopf surfaces can be constructed with underlying space a product of a spherical space forms and a circle.

References

Barth, Wolf P.; Hulek, Klaus; Peters, Chris A.M.; Van de Ven, Antonius (2004), Compact Complex Surfaces, Ergebnisse der Mathematik und ihrer Grenzgebiete. 3. Folge. 4, Springer-Verlag, Berlin, ISBN 978-3-540-00832-3, MR 2030225

Hopf, Heinz (1948), "Zur Topologie der komplexen Mannigfaltigkeiten", Studies and Essays Presented to R. Courant on his 60th Birthday, January 8, 1948, Interscience Publishers, Inc., New York, pp. 167–185, MR 0023054

Kato, Masahide (1975), "Topology of Hopf surfaces", Journal of the Mathematical Society of Japan 27 (2): 222–238, doi:10.2969/jmsj/02720222, ISSN 0025-5645, MR 0402128 Kato, Masahide (1989), "Erratum to: "Topology of Hopf surfaces"", Journal of the Mathematical Society of Japan 41 (1): 173–174, doi:10.2969/jmsj/04110173, ISSN 0025-5645, MR 972171

Kodaira, Kunihiko (1966), "On the structure of compact complex analytic surfaces. II", American Journal of Mathematics (The Johns Hopkins University Press) 88 (3): 682–721, doi:10.2307/2373150, ISSN 0002-9327, JSTOR 2373150, MR 0205280

Kodaira, Kunihiko (1968), "On the structure of compact complex analytic surfaces. III", American Journal of Mathematics (The Johns Hopkins University Press) 90 (1): 55–83, doi:10.2307/2373426, ISSN 0002-9327, JSTOR 2373426, MR 0228019

Kodaira, Kunihiko (1966b), "Complex structures on S1×S3" (PDF), Proceedings of the National Academy of Sciences of the United States of America 55 (2): 240–243, doi:10.1073/pnas.55.2.240, ISSN 0027-8424, MR 0196769

Matumoto, Takao; Nakagawa, Noriaki (2000), "Explicit description of Hopf surfaces and their automorphism groups", Osaka Journal of Mathematics 37 (2): 417–424, ISSN 0030-6126, MR 1772841

Ornea, L. (2001), "Hopf manifold", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4

Undergraduate Texts in Mathematics

Graduate Studies in Mathematics

Retrieved from "http://en.wikipedia.org/"

All text is available under the terms of the GNU Free Documentation License