Linear system of divisors

In algebraic geometry, a linear system of divisors is an algebraic generalization of the geometric notion of a family of curves; the dimension of the linear system corresponds to the number of parameters of the family. These arose first in the form of a linear system of algebraic curves in the projective plane. It assumed a more general form, through gradual generalisation, so that one could speak of linear equivalence of divisors D on a general scheme or even a ringed space (X, OX). Linear system of dimension 1, 2, or 3 are called a pencil, a net, or a web, respectively. A map determined by a linear system is sometimes called the Kodaira map. Given a general variety , two divisors are linearly equivalent if for some non-zero rational function on , or in other words a non-zero element of the function field . Here denotes the divisor of zeroes and poles of the function . Note that if has singular points, the notion of 'divisor' is inherently ambiguous (Cartier divisors, Weil divisors: see divisor (algebraic geometry)). The definition in that case is usually said with greater care (using invertible sheaves or holomorphic line bundles); see below. A complete linear system on is defined as the set of all effective divisors linearly equivalent to some given divisor . It is denoted . Let be the line bundle associated to . In the case that is a nonsingular projective variety, the set is in natural bijection with by associating the element of to the set of non-zero multiples of (this is well defined since two non-zero rational functions have the same divisor if and only if they are non-zero multiples of each other). A complete linear system is therefore a projective space. A linear system is then a projective subspace of a complete linear system, so it corresponds to a vector subspace W of The dimension of the linear system is its dimension as a projective space. Hence . Linear systems can also be introduced by means of the line bundle or invertible sheaf language.