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Conic bundle

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In algebraic geometry, a conic bundle is an algebraic variety that appears as a solution of a Cartesian equation of the form:

Conic bundles can be considered as a Severi–Brauer surface, or, more precisely, a Châtelet surface. This can be a double covering of a ruled surface. Through an isomorphism, it can be associated with the symbol in the second Galois cohomology of the field . In practice, it is more commonly observed as a surface with a well-understood divisor class group, and the simplest cases share with Del Pezzo surfaces the property of being a rational surface. But many problems of contemporary mathematics remain open, notably (for those examples which are not rational) the question of unirationality.

A point of view

In order to properly express a conic bundle, the initial step involves simplifying the quadratic form on the left side. This can be achieved through an alteration, as such:

In a second step, it should be placed in a projective space in order to complete the surface at infinity.

To achieve this, we write the equation in homogeneous coordinates and express the first visible part of the fiber:

That is not enough to complete the fiber as non-singular (smooth and proper), and then glue it to infinity by a change of classical maps.

Seen from infinity, (i.e. through the change ), the same fiber (excepted the fibers and ), written as the set of solutions where appears naturally as the reciprocal polynomial of . Details are below about the map-change .

The fiber c

Going a little further, while simplifying the issue, limit to cases where the field is of characteristic zero and denote by any integer except zero. Denote by P(T) a polynomial with coefficients in the field , of degree 2m or 2m − 1, without multiple roots. Consider the scalar a.

One defines the reciprocal polynomial by , and the conic bundle Fa,P as follows:

Definition

is the surface obtained as "glueing" of the two surfaces and of equations

and

along the open sets by Isomorphism

and .

One shows the following result:

Fundamental property

The surface Fa,P is a k smooth and proper surface, the mapping defined by

by

and the same on gives to Fa,P a structure of conic bundle over P1,k.

See also

References

  • Robin Hartshorne (1977). Algebraic Geometry. Springer-Verlag. ISBN 0-387-90244-9.
  • David Cox; John Little; Don O'Shea (1997). Ideals, Varieties, and Algorithms (second ed.). Springer-Verlag. ISBN 0-387-94680-2.
  • David Eisenbud (1999). Commutative Algebra with a View Toward Algebraic Geometry. Springer-Verlag. ISBN 0-387-94269-6.