The Moduli Space of (1,11)-Polarized Abelian Surfaces is Unirational

We prove that the moduli space A ₁₁ ^lev of (1,11)-polarized Abelian surfaces with level structure of canonical type is birational to Klein's cubic hypersurface in P ⁴ . Therefore, A ₁₁ ^lev is unirational but not rational, and there are no ₁₁-cusp forms of weight 3. The same methods also provide an easy proof of the rationality of A ₉ ^lev .     

Real Quotient Singularities and Nonsingular Real Algebraic Curves in the Boundary of the Moduli Space

The quotient of a real analytic manifold by a properly discontinuous group action is, in general, only a semianalytic variety. We study the boundary of such a quotient, i.e., the set of points at which the quotient is not analytic. We apply the results to the moduli space M_g/ of nonsingular real algebraic curves of genus g (g2). This moduli space has a natural structure of a semianalytic variety. We determine the dimension of the boundary of any connected component of M_g/. It turns out that every connected component has a nonempty boundary. In particular, no connected component of M_g/ is real analytic. We conclude that M_g/ is not a real analytic variety.     

The Moduli Space of Six-Dimensional Two-Step Nilpotent Lie Algebras

We determine the moduli space of metric two-step nilpotent Lie algebras of dimension up to 6. This space is homeomorphic to a cone over a four-dimensional contractible simplicial complex. Moreover, we exhibit standard metric representatives of the seven isomorphism types of six-dimensional two-step nilpotent Lie algebras within our picture. Mathematics Subject Classifications (2000): Primary 22E25, 53C30, 22E60     

Galois Theory for the Selmer Group of an Abelian Variety

This paper concerns the Galois theoretic behavior of the p-primary subgroup Sel _A (F) _p of the Selmer group for an Abelian variety A defined over a number field F in an extension K/F such that the Galois group G(K/F) is a p-adic Lie group. Here p is any prime such that A has potentially good, ordinary reduction at all primes of F lying above p. The principal results concern the kernel and the cokernel of the natural map s _K/F Sel _A (F) _p Sel _A (K) _p ^G(K/F) where F is any finite extension of F contained in K. Under various hypotheses on the extension K/F, it is proved that the kernel and cokernel are finite. More precise results about their structure are also obtained. The results are generalizations of theorems of B.Mazurand M. Harris.     

The de Rham Bundle on a Compactification of Moduli Space of Abelian Varieties

In this paper, we show that the Chern classes c _k of the de Rham bundle defined on any good toroidal compactification of the moduli space of Abelian varieties of dimension g are zero in the rational Chow ring of , for g=4, 5 and k>0.     

Albanese homomorphism of the Chow group of 0-cycles of a real Algebraic variety

We study a certain homomorphism of the Chow group of 0-cycles of degree zero of a real algebraic variety into the group of real points of the Albanese variety; this homomorphism is obtained from the Albanese mapping for the corresponding variety. The kernel of this homomorphism is calculated and estimates for the kernel of the mapping of the torsion groups are obtained. Translated fromMatematicheskie Zametki, Vol. 65, No. 1, pp. 76–83, January, 1999.     

The Jacobson Radical of an Endomorphism Ring for an Abelian Group

The Jacobson radical of an endomorphism ring is computed for a completely decomposable torsion-free Abelian group and for a mixed Abelian group in one class of mixed groups. For the latter case, we also look into the question when a factor ring w.r.t. the radical is regular in the sense of Nuemann.     

Moduli Problems of Sheaves Associated with Oriented Trees

To every oriented tree we associate vector bundle problems. We define semistability concepts for these vector bundle problems and establish the existence of moduli spaces. As an important application, we obtain an algebraic construction of the moduli space of holomorphic triples.     

实无限维线性空间中的较多序

本文引入实无限维线性空间中的较多锥和严格较多锥,利用它们定义较多序,讨论较多序的性质,由此得出,实无限维线性空间中的任何两个元素都可以按较多序进行比较．     

The Holomorphic 2-Number of a Hermitian Symmetric Space

This paper contains a proof a priori (i.e. independent of the classification of Hermitian symmetric spaces) of a theorem on the holomorphic 2-number of a Hermitian symmetric space. If N=G/K is a Hermitian symmetric space, where G is a compact simply connected simple Lie group, T a maximal torus of G and F(T,N) = E₁,... , E_m is the fixed point set of T in N, then for each pair E_i, E_j there is a two-dimensional sphere N_ij N such that E_i and E_j are antipodal points of N_ij.     

The Absolute of a Topological Space and its Application to Abelian l-groups

We give a axiomatic treatment of the absolute in the category of all topological spaces and characterize it as a cover with respect to the full subcategory of extremally disconnected spaces. As an application, we obtain the strongly projectable hull of an abelian l-group as a unique lifting of the absolute of its spectrum. Dedicated to B. V. M.     

Dynamics of the Mapping Class Group on the Moduli of a Punctured Sphere with Rational Holonomy

Let M be a four-holed sphere and Γ the mapping class group of M fixing the boundary ∂M. The group Γ acts on which is the space of completely reducible SL (2, -gauge equivalence classes of flat SL -connections on M with fixed holonomy on ∂M. Let and be the compact component of the real points of . These points correspond to SU(2)-representations or SL(2, -representations. The Γ-action preserves and we study the topological dynamics of the Γ-action on and show that for a dense set of holonomy , the Γ-orbits are dense in . We also produce a class of representations such that the Γ-orbit of [ρ] is finite in the compact component of , but is dense in SL(2, .Mathematics Subject Classiffications (2000). 57M05, 54H20, 11D99     

Orbits and Invariants Associated with a Pair of Spherical Varieties: Some Examples

Let H and K be spherical subgroups of a reductive complex group G. In many cases, detailed knowledge of the double coset space H\G/K is of fundamental importance in group theory and representation theory. If H or K is parabolic, then H\G/K is finite, and we recall the classification of the double cosets in several important cases. If H=K is a symmetric subgroup of G, then the double coset space K\G/K (and the corresponding invariant theoretic quotient) are no longer finite, but several nice properties hold, including an analogue of the Chevalley restriction theorem. These properties were generalized by Helminck and Schwarz (Duke Math. J. 106(2) (2001), pp. 237–279) to the case where H and K are fixed point groups of commuting involutions. We recall Helminck and Schwarz's main results. We also give examples to show the difficulty in extending these results if we allow H=K to be a reductive spherical (nonsymmetric) subgroup or if we have H symmetric and K spherical reductive.     

The Brauer and Witt Groups of Real Ruled Surfaces

The Brauer group and the Witt group of a real irrational ruled surface are determined by using equivariant cohomology.     

The failure of spectral synthesis on some types of discrete Abelian groups

The problem of spectral synthesis on arbitrary Abelian groups is solved in the negative.     

An Investigation of the Fields of Bounded Formal Power Series by Means of Theory of Cuts

We consider a class K of real closed fields F, |F|=|G|=₁, where G is a group of Archimedean classes of F, and cofinality of each symmetric gap of F is ₁. We will show that this class is exactly a class of all bounded formal power series RG,₁, where G is a divisible Abelian group, card(G)=₁, under CH. A nonstandard real line ^*R, which is ₁-set belongs to this class; we will also consider a construction RG(L,P),₁ of fields from this class, where L is a totally ordered set, P is a totally ordered field, G(L,P) is a group of finite words. It will be describes symmetric gaps of such two fields in K, which are not ₁-set. Mathematics Subject Classifications (2000) 03E04, 12J15, 12J25.The work was supported by grant of Ministry of Education PD02-1.1-386.     

A Foliation of the Space of Conjugacy Classes of Representations of a Surface Group

Let be the fundamental group of a closed orientable surface of genus g 1, and let R(, G)/G be the space of conjugacy classes of representations of into a connected real reductive Lie group G. Motivated by the theory of geometric quantization, we define a map ¯ on R(, G)/G and investigate whether the fibres of ¯ are isotropic with respect to the natural symplectic structure on R(, G)/G. If g = 2 and G = SU(2), then the foliation given by the fibres of ¯ is equivalent to a real polarization defined by Weitsman, and we reprove his result that the fibres are isotropic in this case. If g = 1 then the fibres of ¯ are also isotropic, but we give an example to show that in general they are not.     

The Brauer group of a noncomplete real algebraic surface

The Brauer group of a noncomplete real algebraic surface is calculated. The calculations make use of equivariant cohomology. The resulting formula is similar to the formula for a complete surface, but the proof is substantially different. Translated fromMatematicheskie Zametki, Vol. 67, No. 3, pp. 355–359, March, 2000.