Linear system

A family of effective linearly equivalent divisors (cf. Divisor) on an algebraic variety, parametrized by projective space.

Let be a non-singular algebraic variety over a field , an invertible sheaf on , the space of global sections of , and a finite-dimensional subspace. If , then the divisors determined by zero sections of are linearly equivalent and effective. A linear system is the projective space of one-dimensional subspaces of that parametrizes these divisors. If , then the linear system is said to be complete; it is denoted by .

Let be a basis of . It defines a rational mapping by the formula

One usually says that is defined by the linear system . The image does not lie in any hyperplane of (see [2]). Conversely, every rational mapping having this property is defined by some linear system.

A fixed component of a linear system is an effective divisor on such that for any , where is an effective divisor. When runs through , the divisors form a linear system of the same dimension as . The mapping coincides with . Therefore, in considering one may assume that does not have fixed components. In this case is not defined exactly on the basic set of .

Examples.

1) Let and , ; then the sections of can be identified with forms of degree on , and the complete linear system can be identified with the set of all curves of order .

2) The standard quadratic transformation (see Cremona transformation) is defined by the linear system of conics passing through the points , , .

3) The Geiser involution is defined by the linear system of curves of order 8 passing with multiplicity 3 through 7 points in general position (cf. Point in general position).

4) The Bertini involution is defined by the linear system of curves of order 17 passing with multiplicity 6 through 8 points in general position.

References

[1]	I.R. Shafarevich, "Algebraic surfaces" Proc. Steklov Inst. Math. , 75 (1967) Trudy Mat. Inst. Steklov. , 75 (1965)
[2]	D. Mumford, "Lectures on curves on an algebraic surface" , Princeton Univ. Press (1966)
[3]	O. Zariski, "Algebraic surfaces" , Springer (1971)

Comments

In classical (elementary) projective and analytic geometry one speaks of linear systems of curves, surfaces, quadrics, etc. These are families of curves, surfaces, etc. of the form

where the define individual curves, surfaces, etc. If the family is one-dimensional (i.e. through a point in general position passes one member of the family), one speaks of a pencil; a two-dimensional family (i.e. two different members of the family pass through a point in general position) is called a net; and a three- (or higher-) dimensional family is called a web, [a1]. Instead of "net" the term "bundlebundle" is also occasionally used and instead of "web" one also sometimes finds "net" .

Quite generally, if is an open subset of , a codimension -web on is defined by foliations of codimension on such that for each the leaves passing through are in general position. Cf. also Web. Especially in the case of a codimension -web, i.e. an -web of curves, on (same ) the word net is often used.

The phrase "linear system" of course also occurs (as an abbreviation) in many other parts of mathematics. E.g. in differential equation theory: for system of linear differential equations, and in control and systems theory: for linear input/output systems, linear dynamical systems or linear control system.

References

[a1]	J.A. Todd, "Projective and analytical geometry" , Pitman (1947) pp. Chapt. VI