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Ample line bundle information


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 a complete variety 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.

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Ample line bundle

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of an ample line bundle, although there are several related classes of line bundles. Roughly speaking, positivity properties of a line bundle are related...

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Line bundle

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a bundle to have no non-zero global sections at all; this is the case for the tautological line bundle. When the line bundle is sufficiently ample this...

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Nef line bundle

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called semi-ample if some positive tensor power L ⊗ a {\displaystyle L^{\otimes a}} is basepoint-free. It follows that a semi-ample line bundle is nef. Semi-ample...

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Moduli space

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{\displaystyle i^{*}x_{0},\ldots ,i^{*}x_{n}} . Conversely, given an ample line bundle L → X {\displaystyle {\mathcal {L}}\to X} globally generated by n...

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Canonical bundle

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anticanonical bundle is the corresponding inverse bundle ω − 1 {\displaystyle \omega ^{-1}} . When the anticanonical bundle of V {\displaystyle V} is ample, V {\displaystyle...

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Quillen metric

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interpretation of the ample line bundle over the moduli space of vector bundles on a compact Riemann surface, known as the Quillen determinant line bundle. It can be...

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Tautological bundle

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tautological bundle is known as the tautological line bundle. The tautological bundle is also called the universal bundle since any vector bundle (over a compact...

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Positive form

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Kodaira embedding theorem claims that a positive line bundle is ample, and conversely, any ample line bundle admits a Hermitian metric with − 1 Θ {\displaystyle...

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Iitaka dimension

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varieties, and if L is a big line bundle on X, then f*L is a big line bundle on Y. All ample line bundles are big. Big line bundles need not determine birational...

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Coherent sheaf

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geometry. For example, the fact that the canonical bundle is a negative multiple of the ample line bundle O ( 1 ) {\displaystyle {\mathcal {O}}(1)} means...

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Algebraic geometry of projective spaces

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canonical line bundle makes projective spaces prime examples of Fano varieties, equivalently, their anticanonical line bundle is ample (in fact very ample). Their...

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Projective variety

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there exists a very ample sheaf on X relative to S. Indeed, if X is proper, then an immersion corresponding to the very ample line bundle is necessarily closed...

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Seshadri constant

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In algebraic geometry, a Seshadri constant is an invariant of an ample line bundle L at a point P on an algebraic variety. It was introduced by Demailly...

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Equivariant sheaf

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linearizations of the trivial line bundle. See Example 2.16 of [1] for an example of a variety for which most line bundles are not linearizable. Given an...

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Equations defining abelian varieties

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third power of an ample line bundle is normally generated. The Mumford–Kempf theorem states that the fourth power of an ample line bundle is quadratically...

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Algebraic surface

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image D ¯ {\displaystyle {\bar {D}}} is abbreviated with D.) For an ample line bundle H on S, the definition { H } ⊥ := { D ∈ N u m ( S ) | D ⋅ H = 0 }...

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K3 surface

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K3 surface together with an ample line bundle L such that L is primitive (that is, not 2 or more times another line bundle) and c 1 ( L ) 2 = 2 g − 2 {\displaystyle...

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Divisorial scheme

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divisorial scheme is a scheme admitting an ample family of line bundles, as opposed to an ample line bundle. In particular, a quasi-projective variety...

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Sheaf of modules

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the theory of schemes, a related notion is ample line bundle. (For example, if L is an ample line bundle, some power of it is generated by global sections...

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Coherent sheaf cohomology

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zero, L {\displaystyle L} is an ample line bundle on X {\displaystyle X} , and K X {\displaystyle K_{X}} a canonical bundle, then H j ( X , K X ⊗ L ) = 0...

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Quot scheme

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Hilbert polynomial Φ {\displaystyle \Phi } . For a relatively very ample line bundle L ∈ Pic ( X ) {\displaystyle {\mathcal {L}}\in {\text{Pic}}(X)} and...

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Complete intersection

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of more than two polynomials. We can construct it using the very ample line bundle O ( 3 ) {\displaystyle {\mathcal {O}}(3)} over P 1 {\displaystyle...

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Theorem of the highest weight

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irreducible representation as the space of global sections of an ample line bundle; the highest weight theorem results as a consequence. (The approach...

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Algebraic variety

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complete toric variety that has no non-trivial line bundle; thus, in particular, it has no ample line bundle. Definition 1.1.12 in Ginzburg, V., 1998. Lectures...

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List of algebraic geometry topics

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Zariski tangent space Function field of an algebraic variety Ample line bundle Ample vector bundle Linear system of divisors Birational geometry Blowing up...

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