Conic bundle

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

$X^{2}+aXY+bY^{2}=P(T).\,$

Theoretically, it can be considered as a Severi–Brauer surface, or more precisely as a Châtelet surface. This can be a double covering of a ruled surface. Through an isomorphism, it can be associated with a symbol $(a,P)$ in the second Galois cohomology of the field $k$.

In fact, it is a surface with a well-understood divisor class group and 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 naive 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 a harmless alteration, resulting in a more straightforward and uncomplicated expression such as:

$X^{2}-aY^{2}=P(T).\,$

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

To do this, we write the equation in homogeneous coordinates and expresses the first visible part of the fiber

$X^{2}-aY^{2}=P(T)Z^{2}.\,$

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 $T\mapsto T'={\frac {1}{T}}$), the same fiber (excepted the fibers $T=0$ and $T'=0$), written as the set of solutions $X'^{2}-aY'^{2}=P^{*}(T')Z'^{2}$ where $P^{*}(T')$ appears naturally as the reciprocal polynomial of $P$. Details are below about the map-change $[x':y':z']$.

The fiber c

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

One defines the reciprocal polynomial by $P^{*}(T')=T^{2m}P({\frac {1}{T}})$, and the conic bundle F_a,P as follows :

Definition

$F_{a,P}$ is the surface obtained as "gluing" of the two surfaces $U$ and $U'$ of equations

$X^{2}-aY^{2}=P(T)Z^{2}$

and

$X'^{2}-aY'^{2}=P(T')Z'^{2}$

along the open sets by isomorphisms

$x'=x,,y'=y,$ and $z'=zt^{m}$.

One shows the following result :

Fundamental property

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

$p:U\to P_{1,k}$

by

$([x:y:z],t)\mapsto t$

and the same on $U'$ gives to F_a,P a structure of conic bundle over P_1,k.

See also

• Algebraic surface
• Intersection number (algebraic geometry)
• List of complex and algebraic surfaces

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.