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 "X" amounts to understanding the different ways of mapping "X" into projective space. 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 "X" is very ample if it has enough sections to give a closed immersion (or "embedding") of "X" into projective space. A line bundle is ample if some positive power is very ample.
An ample line bundle on a projective variety "X" has positive degree on every curve in "X". The converse is not quite true, but there are corrected versions of the converse, the Nakai–Moishezon and Kleiman criteria for ampleness.
Introduction.
Pullback of a line bundle and hyperplane divisors.
Given a morphism formula_1 of schemes, a vector bundle "E" on "Y" (or more generally a coherent sheaf on "Y") has a pullback to "X", formula_2 (see Sheaf of modules#Operations). The pullback of a vector bundle is a vector bundle of the same rank. In particular, the pullback of a line bundle is a line bundle. (Briefly, the fiber of formula_2 at a point "x" in "X" is the fiber of "E" at "f"("x").)
The notions described in this article are related to this construction in the case of a morphism to projective space
with "E" = "O"(1) the line bundle on projective space whose global sections are the homogeneous polynomials of degree 1 (that is, linear functions) in variables formula_5. The line bundle "O"(1) can also be described as the line bundle associated to a hyperplane in formula_6 (because the zero set of a section of "O"(1) is a hyperplane). If "f" is a closed immersion, for example, it follows that the pullback formula_7 is the line bundle on "X" associated to a hyperplane section (the intersection of "X" with a hyperplane in formula_8).
Basepoint-free line bundles.
Let "X" be a scheme over a field "k" (for example, an algebraic variety) with a line bundle "L". (A line bundle may also be called an invertible sheaf.) Let formula_9 be elements of the "k"-vector space formula_10 of global sections of "L". The zero set of each section is a closed subset of "X"; let "U" be the open subset of points at which at least one of formula_11 is not zero. Then these sections define a morphism
In more detail: for each point "x" of "U", the fiber of "L" over "x" is a 1-dimensional vector space over the residue field "k"("x"). Choosing a basis for this fiber makes formula_13 into a sequence of "n"+1 numbers, not all zero, and hence a point in projective space. Changing the choice of basis scales all the numbers by the same nonzero constant, and so the point in projective space is independent of the choice.
Moreover, this morphism has the property that the restriction of "L" to "U" is isomorphic to the pullback formula_7.
The base locus of a line bundle "L" on a scheme "X" is the intersection of the zero sets of all global sections of "L". A line bundle "L" is called basepoint-free if its base locus is empty. That is, for every point "x" of "X" there is a global section of "L" which is nonzero at "x". If "X" is proper over a field "k", then the vector space formula_10 of global sections has finite dimension; the dimension is called formula_16. So a basepoint-free line bundle "L" determines a morphism formula_17 over "k", where formula_18, given by choosing a basis for formula_10. Without making a choice, this can be described as the morphism
from "X" to the space of hyperplanes in formula_10, canonically associated to the basepoint-free line bundle "L". This morphism has the property that "L" is the pullback formula_7.
Conversely, for any morphism "f" from a scheme "X" to projective space formula_8 over "k", the pullback line bundle formula_7 is basepoint-free. Indeed, "O"(1) is basepoint-free on formula_8, because for every point "y" in formula_8 there is a hyperplane not containing "y". Therefore, for every point "x" in "X", there is a section "s" of "O"(1) over formula_8 that is not zero at "f"("x"), and the pullback of "s" is a global section of formula_7 that is not zero at "x". In short, basepoint-free line bundles are exactly those that can be expressed as the pullback of "O"(1) by some morphism to projective space.
Nef, globally generated, semi-ample.
The degree of a line bundle "L" on a proper curve "C" over "k" is defined as the degree of the divisor ("s") of any nonzero rational section "s" of "L". The coefficients of this divisor are positive at points where "s" vanishes and negative where "s" has a pole. Therefore, any line bundle "L" on a curve "C" such that formula_29 has nonnegative degree (because sections of "L" over "C", as opposed to rational sections, have no poles). In particular, every basepoint-free line bundle on a curve has nonnegative degree. As a result, a basepoint-free line bundle "L" on any proper scheme "X" over a field is nef, meaning that "L" has nonnegative degree on every (irreducible) curve in "X".
More generally, a sheaf "F" of formula_30-modules on a scheme "X" is said to be globally generated if there is a set "I" of global sections formula_31 such that the corresponding morphism
of sheaves is surjective. A line bundle is globally generated if and only if it is basepoint-free.
For example, every quasi-coherent sheaf on an affine scheme is globally generated. Analogously, in complex geometry, Cartan's theorem A says that every coherent sheaf on a Stein manifold is globally generated.
A line bundle "L" on a proper scheme over a field is semi-ample if there is a positive integer "r" such that the tensor power formula_33 is basepoint-free. A semi-ample line bundle is nef (by the corresponding fact for basepoint-free line bundles).
Very ample line bundles.
A line bundle "L" on a proper scheme "X" over a field "k" is said to be very ample if it is basepoint-free and the associated morphism
is a closed immersion. Here formula_18. Equivalently, "L" is very ample if "X" can be embedded into projective space of some dimension over "k" in such a way that "L" is the restriction of the line bundle "O"(1) to "X". The latter definition is used to define very ampleness for a line bundle on a proper scheme over any commutative ring.
The name "very ample" was introduced by Alexander Grothendieck in 1961. Various names had been used earlier in the context of linear systems of divisors.
For a very ample line bundle "L" on a proper scheme "X" over a field with associated morphism "f", the degree of "L" on a curve "C" in "X" is the degree of "f"("C") as a curve in formula_8. So "L" has positive degree on every curve in "X" (because every subvariety of projective space has positive degree).
Definitions.
A line bundle "L" on a proper scheme "X" over a commutative ring "R" is said to be ample if there is a positive integer "r" such that the tensor power formula_33 is very ample. In particular, a proper scheme over "R" has an ample line bundle if and only if it is projective over "R". An ample line bundle on a proper scheme "X" over a field has positive degree on every curve in "X", by the corresponding statement for very ample line bundles.
A Cartier divisor "D" on a proper scheme "X" over a field "k" is said to be ample if the corresponding line bundle "O"("D") is ample. (For example, if "X" is smooth over "k", then a Cartier divisor can be identified with a finite linear combination of closed codimension-1 subvarieties of "X" with integer coefficients.)
On an arbitrary scheme "X", Grothendieck defined a line bundle "L" to be ample if "X" is quasi-compact and for every point "x" in "X" there is a positive integer "r" and a section formula_38 such that "s" is nonzero at "x" and the open subscheme formula_39 is affine. For example, the trivial line bundle formula_30 is ample if and only if "X" is quasi-affine. The rest of this article will concentrate on ampleness on proper schemes over a field.
Weakening the notion of "very ample" to "ample" gives a flexible concept with a wide variety of different characterizations. A first point is that tensoring high powers of an ample line bundle with any coherent sheaf whatsoever gives a sheaf with many global sections. More precisely, a line bundle "L" on a proper scheme "X" over a field (or more generally over a Noetherian ring) is ample if and only if for every coherent sheaf "F" on "X", there is an integer "s" such that the sheaf formula_41 is globally generated for all formula_42. Here "s" may depend on "F".
Another characterization of ampleness, known as the Cartan–Serre–Grothendieck theorem, is in terms of coherent sheaf cohomology. Namely, a line bundle "L" on a proper scheme "X" over a field (or more generally over a Noetherian ring) is ample if and only if for every coherent sheaf "F" on "X", there is an integer "s" such that
for all formula_44 and all formula_42. In particular, high powers of an ample line bundle kill cohomology in positive degrees. This implication is called the Serre vanishing theorem, proved by Jean-Pierre Serre in his 1955 paper Faisceaux algébriques cohérents.
Criteria for ampleness of line bundles.
Intersection theory.
To determine whether a given line bundle on a projective variety "X" is ample, the following "numerical criteria" (in terms of intersection numbers) are often the most useful. It is equivalent to ask when a Cartier divisor "D" on "X" is ample, meaning that the associated line bundle "O"("D") is ample. The intersection number formula_64 can be defined as the degree of the line bundle "O"("D") restricted to "C". In the other direction, for a line bundle "L" on a projective variety, the first Chern class formula_65 means the associated Cartier divisor (defined up to linear equivalence), the divisor of any nonzero rational section of "L".
On a smooth projective curve "X" over an algebraically closed field "k", a line bundle "L" is very ample if and only if formula_66 for all "k"-rational points "x","y" in "X". Let "g" be the genus of "X". By the Riemann–Roch theorem, every line bundle of degree at least 2"g" + 1 satisfies this condition and hence is very ample. As a result, a line bundle on a curve is ample if and only if it has positive degree.
For example, the canonical bundle formula_67 of a curve "X" has degree 2"g" − 2, and so it is ample if and only if formula_68. The curves with ample canonical bundle form an important class; for example, over the complex numbers, these are the curves with a metric of negative curvature. The canonical bundle is very ample if and only if formula_68 and the curve is not hyperelliptic.
The Nakai–Moishezon criterion (named for Yoshikazu Nakai (1963) and Boris Moishezon (1964)) states that a line bundle "L" on a proper scheme "X" over a field is ample if and only if formula_70 for every (irreducible) closed subvariety "Y" of "X" ("Y" is not allowed to be a point). In terms of divisors, a Cartier divisor "D" is ample if and only if formula_71 for every (nonzero-dimensional) subvariety "Y" of "X". For "X" a curve, this says that a divisor is ample if and only if it has positive degree. For "X" a surface, the criterion says that a divisor "D" is ample if and only if its self-intersection number formula_72 is positive and every curve "C" on "X" has formula_73.
Kleiman's criterion.
To state Kleiman's criterion (1966), let "X" be a projective scheme over a field. Let formula_74 be the real vector space of 1-cycles (real linear combinations of curves in "X") modulo numerical equivalence, meaning that two 1-cycles "A" and "B" are equal in formula_74 if and only if every line bundle has the same degree on "A" and on "B". By the Néron–Severi theorem, the real vector space formula_74 has finite dimension. Kleiman's criterion states that a line bundle "L" on "X" is ample if and only if "L" has positive degree on every nonzero element "C" of the closure of the cone of curves NE("X") in formula_74. (This is slightly stronger than saying that "L" has positive degree on every curve.) Equivalently, a line bundle is ample if and only if its class in the dual vector space formula_78 is in the interior of the nef cone.
Kleiman's criterion fails in general for proper (rather than projective) schemes "X" over a field, although it holds if "X" is smooth or more generally Q-factorial.
A line bundle on a projective variety is called strictly nef if it has positive degree on every curve. and David Mumford constructed line bundles on smooth projective surfaces that are strictly nef but not ample. This shows that the condition formula_79 cannot be omitted in the Nakai–Moishezon criterion, and it is necessary to use the closure of NE("X") rather than NE("X") in Kleiman's criterion. Every nef line bundle on a surface has formula_80, and Nagata and Mumford's examples have formula_81.
C. S. Seshadri showed that a line bundle "L" on a proper scheme over an algebraically closed field is ample if and only if there is a positive real number ε such that deg("L"|"C") ≥ ε"m"("C") for all (irreducible) curves "C" in "X", where "m"("C") is the maximum of the multiplicities at the points of "C".
Several characterizations of ampleness hold more generally for line bundles on a proper algebraic space over a field "k". In particular, the Nakai-Moishezon criterion is valid in that generality. The Cartan-Serre-Grothendieck criterion holds even more generally, for a proper algebraic space over a Noetherian ring "R". (If a proper algebraic space over "R" has an ample line bundle, then it is in fact a projective scheme over "R".) Kleiman's criterion fails for proper algebraic spaces "X" over a field, even if "X" is smooth.
Openness of ampleness.
On a projective scheme "X" over a field, Kleiman's criterion implies that ampleness is an open condition on the class of an R-divisor (an R-linear combination of Cartier divisors) in formula_78, with its topology based on the topology of the real numbers. (An R-divisor is defined to be ample if it can be written as a positive linear combination of ample Cartier divisors.) An elementary special case is: for an ample divisor "H" and any divisor "E", there is a positive real number "b" such that formula_83 is ample for all real numbers "a" of absolute value less than "b". In terms of divisors with integer coefficients (or line bundles), this means that "nH" + "E" is ample for all sufficiently large positive integers "n".
Ampleness is also an open condition in a quite different sense, when the variety or line bundle is varied in an algebraic family. Namely, let formula_84 be a proper morphism of schemes, and let "L" be a line bundle on "X". Then the set of points "y" in "Y" such that "L" is ample on the fiber formula_85 is open (in the Zariski topology). More strongly, if "L" is ample on one fiber formula_85, then there is an affine open neighborhood "U" of "y" such that "L" is ample on formula_87 over "U".
Kleiman's other characterizations of ampleness.
Kleiman also proved the following characterizations of ampleness, which can be viewed as intermediate steps between the definition of ampleness and numerical criteria. Namely, for a line bundle "L" on a proper scheme "X" over a field, the following are equivalent:
Generalizations.
Ample vector bundles.
Robin Hartshorne defined a vector bundle "F" on a projective scheme "X" over a field to be ample if the line bundle formula_93 on the space formula_94 of hyperplanes in "F" is ample.
Several properties of ample line bundles extend to ample vector bundles. For example, a vector bundle "F" is ample if and only if high symmetric powers of "F" kill the cohomology formula_95 of coherent sheaves for all formula_44. Also, the Chern class formula_97 of an ample vector bundle has positive degree on every "r"-dimensional subvariety of "X", for formula_98.
Big line bundles.
A useful weakening of ampleness, notably in birational geometry, is the notion of a big line bundle. A line bundle "L" on a projective variety "X" of dimension "n" over a field is said to be big if there is a positive real number "a" and a positive integer formula_99 such that formula_100 for all formula_101. This is the maximum possible growth rate for the spaces of sections of powers of "L", in the sense that for every line bundle "L" on "X" there is a positive number "b" with formula_102 for all "j" > 0.
There are several other characterizations of big line bundles. First, a line bundle is big if and only if there is a positive integer "r" such that the rational map from "X" to formula_103 given by the sections of formula_33 is birational onto its image. Also, a line bundle "L" is big if and only if it has a positive tensor power which is the tensor product of an ample line bundle "A" and an effective line bundle "B" (meaning that formula_105). Finally, a line bundle is big if and only if its class in formula_78 is in the interior of the cone of effective divisors.
Bigness can be viewed as a birationally invariant analog of ampleness. For example, if formula_84 is a dominant rational map between smooth projective varieties of the same dimension, then the pullback of a big line bundle on "Y" is big on "X". (At first sight, the pullback is only a line bundle on the open subset of "X" where "f" is a morphism, but this extends uniquely to a line bundle on all of "X".) For ample line bundles, one can only say that the pullback of an ample line bundle by a finite morphism is ample.
Example: Let "X" be the blow-up of the projective plane formula_61 at a point over the complex numbers. Let "H" be the pullback to "X" of a line on formula_61, and let "E" be the exceptional curve of the blow-up formula_110. Then the divisor "H" + "E" is big but not ample (or even nef) on "X", because
This negativity also implies that the base locus of "H" + "E" (or of any positive multiple) contains the curve "E". In fact, this base locus is equal to "E".
Relative ampleness.
Given a quasi-compact morphism of schemes formula_112, an invertible sheaf "L" on "X" is said to be ample relative to "f" or "f"-ample if the following equivalent conditions are met:
The condition 2 says (roughly) that "X" can be openly compactified to a projective scheme with formula_117 (not just to a proper scheme).