Linear system of conics


In algebraic geometry, the conic sections in the projective plane form a linear system of dimension five, as one sees by counting the constants in the degree two equations. The condition to pass through a given point P imposes a single linear condition, so that conics C through P form a linear system of dimension 4. Other types of condition that are of interest include tangency to a given line L.
In the most elementary treatments a linear system appears in the form of equations
with λ and μ unknown scalars, not both zero. Here C and C′ are given conics. Abstractly we can say that this is a projective line in the space of all conics, on which we take
as homogeneous coordinates. Geometrically we notice that any point Q common to C and C′ is also on each of the conics of the linear system. According to Bézout's theorem C and C′ will intersect in four points. Assuming these are in general position, i.e. four distinct intersections, we get another interpretation of the linear system as the conics passing through the four given points. Note that of these conics, exactly three are degenerate, each consisting of a pair of lines, corresponding to the ways of choosing 2 pairs of points from 4 points.

Applications

A striking application of such a family is in which gives a geometric solution to a quartic equation by considering the pencil of conics through the four roots of the quartic, and identifying the three degenerate conics with the three roots of the resolvent cubic.

Example

For example, given the four points the pencil of conics through them can be parameterized as which are the affine combinations of the equations and corresponding to the parallel vertical lines and horizontal lines; this yields degenerate conics at the standard points of A less elegant but more symmetric parametrization is given by in which case inverting a interchanges x and y, yielding the following pencil; in all cases the center is at the origin:
In the terminology of, this is a Type I linear system of conics, and is animated in the linked video.

Classification

There are 8 types of linear systems of conics over the complex numbers, depending on intersection multiplicity at the base points, which divide into 13 types over the real numbers, depending on whether the base points are real or imaginary; this is discussed in and illustrated in.