- Algebraic surface
mathematics, an algebraic surface is an algebraic varietyof dimension two. In the case of geometry over the field of complex numbers, 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
* 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:
* 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 fieldisomorphic to that of the projective plane, being the rational functions in two indeterminates. The cartesian product of two curves also provides examples.
birational geometryof 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 dimension2, 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 numberinvariants 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 upcan add whole curves, with classes in "H"1,1. It is known that Hodge cycles are algebraic, and that algebraic equivalencecoincides 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'.
Riemann-Roch theoremfor 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.
* [http://www.freigeist.cc/gallery.html A gallery of algebraic surfaces]
* [http://www.singsurf.org/singsurf/SingSurf.html SingSurf] an interactive 3D viewer for algebraic surfaces.
* [http://www.mathematik.uni-kl.de/%7Ehunt/drawings.html Some beautiful algebraic surfaces]
* [http://www1-c703.uibk.ac.at/mathematik/project/bildergalerie/gallery.html Some more, with their respective ecuations]
* [http://www.bru.hlphys.jku.at/surf/index.html New Page on Algebraic Surfaces started in June 2008]
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