Ample line bundle

In mathematics, a distinctive feature of algebraic geometry is that some line bundles on a projective variety can be considered "positive", while others are "negative" (or a mixture of the two). The most important notion of positivity is that of an ample line bundle, although there are several related classes of line bundles. Roughly speaking, positivity properties of a line bundle are related to having many global sections. Understanding the ample line bundles on a given variety ${\displaystyle X}$ amounts to understanding the different ways of mapping ${\displaystyle X}$ into projective spaces. In view of the correspondence between line bundles and divisors (built from codimension-1 subvarieties), there is an equivalent notion of an ample divisor.

In more detail, a line bundle is called basepoint-free if it has enough sections to give a morphism to projective space. A line bundle is semi-ample if some positive power of it is basepoint-free; semi-ampleness is a kind of "nonnegativity". More strongly, a line bundle on a complete variety ${\displaystyle X}$ is very ample if it has enough sections to give a closed immersion (or "embedding") of ${\displaystyle X}$ into a projective space. A line bundle is ample if some positive power is very ample.

An ample line bundle on a projective variety ${\displaystyle X}$ has positive degree on every curve in ${\displaystyle X}$. The converse is not quite true, but there are corrected versions of the converse, the Nakai–Moishezon and Kleiman criteria for ampleness.