一般型3维代数簇上的4典范映射

The aim of this paper is to show that the 4-canonical maps of 3-folds of general type is birational when pg≥7 under some given conditions.     

The algebraic dimension of compact complex threefolds with vanishing second Betti number

This note investigates compact complex manifolds X of dimension 3 with second Betti number b₂(X) = 0. If X admits a non-constant meromorphic function, then we prove that either b₁(X) = 1 and b₃(X) = 0 or that b₁(X) = 0 and b₃(X) = 2. The main idea is to show that c₃(X) = 0 by means of a vanishing theorem for generic line bundles on X. As a consequence a compact complex threefold homeomorphic to the 6-sphere S⁶ cannot admit a non-constant meromorphic function. Furthermore we investigate the structure of threefolds with b₂(X) = 0 and algebraic dimension 1, in the case when the algebraic reduction X P₁ is holomorphic.     

A characteristic-free criterion of birationality

One develops ab initio the theory of rational/birational maps over reduced, but not necessarily irreducible, projective varieties in arbitrary characteristic. A virtual numerical invariant of a rational map is introduced, called the Jacobian dual rank. It is proved that a rational map in this general setup is birational if and only if the Jacobian dual rank is well defined and attains its maximal possible value. Even in the “classical” case where the source variety is irreducible there is some gain for this invariant over the degree of the map because, on one hand, it relates naturally to constructs in commutative algebra and, on the other hand, is effectively computable. Applications are given to results only known so far in characteristic zero. One curious byproduct is an alternative approach to deal with the result of Dolgachev concerning the degree of a plane polar Cremona map.     

On the projectivity of threefolds

Let be a smooth complete three-dimensional algebraic variety (defined over an algebraically closed field ). We show that is projective if it contains a divisor which is positive on the cone of effective curves.

     

Characterization of algebraic curves by Chebyshev quadrature

Complex potential theory is used to show that Chebyshev-type quadrature works particularly well on algebraic Jordan curves Γ in ℝ ^d , supplied with normalized arc length or a similar probability measure μ. Evaluating the integral ∫_Γ fdμ by the arithmetic mean of the value off on any cycle ofN equally spaced nodes on Γ (relative to μ), the quadrature error will, be bounded byAe ^−bN sup_Γ|f| for allN and all polynomialsf(x) of degree ≤cN. It is plausible that small shifts of the nodes would give quadrature error zero for such polynomials. There are related results for algebraic Jordan arcs and certain algebraic surfaces. The situation is completely different for nonalgebraic curves and surfaces, where corresponding quadrature remainders are at least of order 1/N.     

Constraints for the normality of monomial subrings and birationality

Let be a field and let be a finite set of monomials whose exponents lie on a positive hyperplane. We give necessary conditions for the normality of both the Rees algebra and the subring . If the monomials in have the same degree, one of the consequences is a criterion for the -rational map defined by to be birational onto its image.

     

On the classification of Hilbert modular threefolds

Letk be a totally real number field with ring of integersO _k . The Hilbert modular variety overk is a desingularization of the (natural) compactification of PSL₂(O _k )∖H ^k . The purpose of this paper is to present specific numerical bounds on the size of the discriminantd _k of a cubic fieldk with Hilbert modular variety of particular classifications. specifically, it is shown that ifd _k>2.12×10⁷, then the Hilbert modular variety overk is not rational and further, ifd _k>2.77×10⁸, then Hilbert modular variety overk is of general type. This material is based on work supported by the National Science Foundation under Grant No. DMS-9008689     

On the Kodaira dimension of minimal threefolds

Dedicated to Professor M. Nagata on his sixtieth birthday     

Derived categories of Fano threefolds

We consider the structure of the derived categories of coherent sheaves on Fano threefolds with Picard number 1 and describe a strange relation between derived categories of different threefolds. In the appendix we discuss how the ring of algebraic cycles of a smooth projective variety is related to the Grothendieck group of its derived category. Published in Russian in Trudy Matematicheskogo Instituta imeni V.A. Steklova, 2009, Vol. 264, pp. 116–128. In memory of V.A. Iskovskikh