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Volume 127 - Issue 1 - May 2001

Contents

Research Article

  • The Tate Conjecture for Cubic Fourfolds over a Finite Field
  • Norman Levin
  • Published online by Cambridge University Press: 04 December 2007, pp. 1-21
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    We prove the Tate conjecture for codimension 2 cycles on an ordinary cubic fourfold over a finite field. The proof involves the construction of canonical coordinates on the formal deformation space via a crystalline period map.

  • On the Density of Elliptic Curves
  • Siman Wong
  • Published online by Cambridge University Press: 04 December 2007, pp. 23-54
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    We show that 17.9% of all elliptic curves over Q, ordered by their exponential height, are semistable, and that there is a positive density subset of elliptic curves for which the root numbers are uniformly distributed. Moreover, for any α > 1/6 (resp. α > 1/12) the set of Frey curves (resp. all elliptic curves) for which the generalized Szpiro Conjecture |Δ(E)| [Lt ]αNE12α is false has density zero. This implies that the ABC Conjecture holds for almost all Frey triples. These results remain true if we use the logarithmic or the Faltings height. The proofs make use of the fibering argument in the square-free sieve of Gouvêa and Mazur. We also obtain conditional as well as unconditional lower bounds for the number of curves with Mordell–Weil rank 0 and [ges ]2, respectively.

  • Actions of Borel Subgroups on Homogeneous Spaces of Reductive Complex Lie Groups and Integrability
  • I. V. Mykytyuk
  • Published online by Cambridge University Press: 04 December 2007, pp. 55-67
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    Let G be a real reductive Lie group, K its compact subgroup. Let A be the algebra of G-invariant real-analytic functions on T*(G/K) (with respect to the Poisson bracket) and let C be the center of A. Denote by 2ε(G,K) the maximal number of functionally independent functions from A\C. We prove that ε(G,K) is equal to the codimension δ(G,K) of maximal dimension orbits of the Borel subgroup B⊂G$^{\Bbb C}$ in the complex algebraic variety G$^{\Bbb C}$/K$^{\Bbb C}$. Moreover, if δ(G,K)=1, then all G-invariant Hamiltonian systems on T*(G/K) are integrable in the class of the integrals generated by the symmetry group G. We also discuss related questions in the geometry of the Borel group action.

  • Holomorphic Functions of Exponential Growth on Abelian Coverings of a Projective Manifold
  • Alexander Brudnyi
  • Published online by Cambridge University Press: 04 December 2007, pp. 69-81
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    Let M be a projective manifold, p: MG→M a regular covering over M with a free Abelian transformation group G. We describe the holomorphic functions on MG of an exponential growth with respect to the distance defined by a metric pulled back from M. As a corollary, we obtain Cartwright and Liouville-type theorems for such functions. Our approach brings together the L2 cohomology technique for holomorphic vector bundles on complete Kähler manifolds and the geometric properties of projective manifolds.

  • Grassmannian Trilogarithms
  • A. B. Goncharov, J. Zhao
  • Published online by Cambridge University Press: 04 December 2007, pp. 83-108
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    In the previous works of the first author, two completely different constructions of single valued Grassmannian trilogarithms were given. One of the constructions, in Math. Res. Lett. 2 (1995), 99–114, is very simple and provides Grassmannian n-logarithms for all n. However its motivic nature is hidden. The other construction in Adv. inMath. 114 (1995), 197–318, is very explicit and motivic, but the generalization for n>4 is not known. In this paper we will compute explicitly the Grassmannian trilogarithm constructed in Math. Res. Lett. 2 (1995), 99–114 and prove that it differs from the motivic Grassmannian trilogarithm by an explicitly given product of logarithms. We also derive some general results about the Grassmannian polylogarithms.

  • Torsion Packets on Curves
  • Matthew Baker, Bjorn Poonen
  • Published online by Cambridge University Press: 04 December 2007, pp. 109-116
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    Let X be a curve of genus g[ges ]2 over a field of characteristic zero. Then X has at most finitely many torsion packets of size greater than 2. Moreover, X has infinitely many torsion packets of size 2 if and only if either g=2, or g=3 and X is both hyperelliptic and bielliptic.