# Algebraic surface

In mathematics, an **algebraic surface** is an algebraic variety of dimension two. In the case of geometry over the complex number field, an algebraic surface is therefore of complex dimension two (as a complex manifold, when it is non-singular) and so of dimension four as a smooth manifold.

The theory of algebraic surfaces is much more complicated than that of algebraic curves (including the compact Riemann surfaces, which are genuine surfaces of (real) dimension two). Many results were obtained, however, in the Italian school of algebraic geometry, and are up to 100 years old.

Examples of algebraic surfaces include (κ is the Kodaira dimension):

- κ=−∞: the projective plane, quadrics in
*P*^{3}, cubic surfaces, Veronese surface, Del Pezzo surfaces, ruled surfaces - κ=0: K3 surfaces, abelian surfaces, Enriques surfaces, hyperelliptic surfaces
- κ=1: Elliptic surfaces
- κ=2: surfaces of general type.

For more examples see the list of algebraic surfaces

The first five examples are in fact birationally equivalent. That is, for example, a cubic surface has a function field isomorphic to that of the projective plane, being the rational functions in two indeterminates. The cartesian product of two curves also provides examples.

The birational geometry of algebraic surfaces is rich, because of blowing up (also known as a monoidal transformation); under which a point is replaced by the *curve* of all limiting tangent directions coming into it (a projective line). Certain curves may also be blown *down*, but there is a restriction (self-intersection number must be −1).

Basic results on algebraic surfaces include the Hodge index theorem, and the division into five groups of birational equivalence classes called the classification of algebraic surfaces. The *general type* class, of Kodaira dimension 2, is very large (degree 5 or larger for a non-singular surface in *P*^{3} lies in it, for example).

There are essential three Hodge number invariants of a surface. Of those, *h*^{1,0} was classically called the **irregularity** and denoted by *q*; and *h*^{2,0} was called the **geometric genus** *p*_{g}. The third, *h*^{1,1}, is not a birational invariant, because blowing up can add whole curves, with classes in *H*^{1,1}. It is known that Hodge cycles are algebraic, and that algebraic equivalence coincides with homological equivalence, so that *h*^{1,1} is an upper bound for ρ, the rank of the Néron-Severi group. The arithmetic genus *p*_{a} is the difference

- geometric genus − irregularity.

In fact this explains why the irregularity got its name, as a kind of 'error term'.

The Riemann-Roch theorem for surfaces was first formulated by Max Noether. The families of curves on surfaces can be classified, in a sense, and give rise to much of their interesting geometry.

## External links

- A gallery of mathematical surfaces
- SingSurf an interactive 3D viewer for algebraic surfaces.
- Some beautiful algebraic surfaces
- Scientific Graphics Project examples of level surfaces.
- Cubic Surface Home Page Images, movies of cubic surface.