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Convexity (algebraic geometry)

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In algebraic geometry, convexity is a restrictive technical condition for algebraic varieties originally introduced to analyze Kontsevich moduli spaces in quantum cohomology.[1]: §1 [2][3] These moduli spaces are smooth orbifolds whenever the target space is convex. A variety is called convex if the pullback of the tangent bundle to a stable rational curve has globally generated sections.[2] Geometrically this implies the curve is free to move around infinitesimally without any obstruction. Convexity is generally phrased as the technical condition

since Serre's vanishing theorem guarantees this sheaf has globally generated sections. Intuitively this means that on a neighborhood of a point, with a vector field in that neighborhood, the local parallel transport can be extended globally. This generalizes the idea of convexity in Euclidean geometry, where given two points in a convex set , all of the points are contained in that set. There is a vector field in a neighborhood of transporting to each point . Since the vector bundle of is trivial, hence globally generated, there is a vector field on such that the equality holds on restriction.

Examples

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There are many examples of convex spaces, including the following.

Spaces with trivial rational curves

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If the only maps from a rational curve to are constants maps, then the pullback of the tangent sheaf is the free sheaf where . These sheaves have trivial non-zero cohomology, and hence they are always convex. In particular, Abelian varieties have this property since the Albanese variety of a rational curve is trivial, and every map from a variety to an Abelian variety factors through the Albanese.[4]

Projective spaces

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Projective spaces are examples of homogeneous spaces, but their convexity can also be proved using a sheaf cohomology computation. Recall the Euler sequence relates the tangent space through a short exact sequence

If we only need to consider degree embeddings, there is a short exact sequence

giving the long exact sequence

since the first two -terms are zero, which follows from being of genus , and the second calculation follows from the Riemann–Roch theorem, we have convexity of . Then, any nodal map can be reduced to this case by considering one of the components of .

Homogeneous spaces

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Another large class of examples are homogenous spaces where is a parabolic subgroup of . These have globally generated sections since acts transitively on , meaning it can take a bases in to a basis in any other point , hence it has globally generated sections.[3] Then, the pullback is always globally generated. This class of examples includes Grassmannians, projective spaces, and flag varieties.

Product spaces

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Also, products of convex spaces are still convex. This follows from the Künneth theorem in coherent sheaf cohomology.

Projective bundles over curves

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One more non-trivial class of examples of convex varieties are projective bundles for an algebraic vector bundle over a smooth algebraic curve[3]pg 6.

Applications

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There are many useful technical advantages of considering moduli spaces of stable curves mapping to convex spaces. That is, the Kontsevich moduli spaces have nice geometric and deformation-theoretic properties.

Deformation theory

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The deformations of in the Hilbert scheme of graphs has tangent space

  [1]

where is the point in the scheme representing the map. Convexity of gives the dimension formula below. In addition, convexity implies all infinitesimal deformations are unobstructed.[5]

Structure

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These spaces are normal projective varieties of pure dimension

  [3]

which are locally the quotient of a smooth variety by a finite group. Also, the open subvariety parameterizing non-singular maps is a smooth fine moduli space. In particular, this implies the stacks are orbifolds.

Boundary divisors

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The moduli spaces have nice boundary divisors for convex varieties given by

  [3]

for a partition of and the point lying along the intersection of two rational curves .

See also

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References

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  1. ^ a b Kontsevich, Maxim (1995). "Enumeration of Rational Curves Via Torus Actions". In Dijkgraaf, Robbert H.; Faber, Carel F.; van der Geer, Gerard B. M. (eds.). The Moduli Space of Curves. Progress in Mathematics. Vol. 129. Boston: Birkhäuser. pp. 335–368. arXiv:hep-th/9405035. doi:10.1007/978-1-4612-4264-2_12. ISBN 978-1-4612-8714-8. S2CID 16131978.
  2. ^ a b Kontsevich, Maxim; Manin, Yuri. "Gromov-Witten Classes, Quantum Cohomology, and Enumerative Geometry" (PDF). p. 9. Archived (PDF) from the original on 2009-11-28.
  3. ^ a b c d e Fulton, W.; Pandharipande, R. (1997-05-17). "Notes on stable maps and quantum cohomology". pp. 6, 12, 29, 31. arXiv:alg-geom/9608011.
  4. ^ "ag.algebraic geometry - Is there any rational curve on an Abelian variety?". MathOverflow. Retrieved 2020-02-28.
  5. ^ Maulik, Davesh. "Lectures on Donaldson-Thomas Theory" (PDF). p. 2. Archived (PDF) from the original on 2020-03-01.
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