Lagrangian subvarieties of Abelian fourfolds
Let (W, ω) be a smooth projective algebraic variety of dimension 2n over C together with a holomorphic (2, 0)-form of maximal rank 2n. A subvariety X ⊂ W is called weakly lagrangian if dim X ≤ n and if the restriction of ω to X is trivial (notice that X can be singular). An n-dimensional subvariety X ⊂ W with this property is called lagrangian. For example, any curve C contained in a K3 or abelian surface S is lagrangian. Further examples of lagrangian subvarieties are obtained by taking a curve C ⊂ S and by considering the corresponding symmetric products. Alternatively, one could look at a product of different curves (of genus > 1) inside a product of abelian varieties. We will say that a variety X ⊂ W is fibered if it admits a dominant map onto a curve of genus > 1. In this note we construct examples of nonfibered lagrangian surfaces in abelian varieties. Our motivation comes from the following Problem 1.1. — Find examples of projective surfaces with a nontrivial fundamental group. In particular, find examples where the fundamental group has a nontrivial nilpotent tower. If X is fibered over a curve C of genus > 1 then the fundamental group π 1 (X) surjects onto a subgroup of finite index in π 1 (C) and consequently both π 1 (X) and its nilpotent tower are big. Therefore, we are interested in examples of surfaces where the nontriviality of π 1 (X) is not induced from curves. Consider the map to the Albanese variety alb : X → Alb(X). One is interested in situations where the natural map H 2 (Alb(X), C) → H 2 (X, C) has a nontrivial kernel – the triviality of the kernel implies the triviality of the nilpotent tower (tensor Q). Such examples were given by Campana ([3], Cor. 1.2) and Sommese-Van de Ven ([7]). However, there the kernel LAGRANGIAN SUBVARIETIES OF ABELIAN FOURFOLDS 3 was found in the map Pic(Alb(X)) → Pic(X). In their construction the fundamental group of the variety is a central extension of an abelian group (and the lower central series has only two steps). The lagrangian property of alb(X) ⊂ Alb(X) implies that there is a nontrivial kernel in H 2,0 (rather than on the level of the Picard groups). We produce an infinite series of surfaces X of different topological types which are …
Shafarevich Maps and Automorphic Forms
This text studies various geometric properties and algebraic invariants of smooth projective varieties with infinite fundamental groups. This approach allows for much interplay between methods of algebraic geometry, complex analysis, the theory of harmonic maps and topology. Making systematic use of Shafarevich maps, this work isolates those varieties where the fundamental group influences global properties of the canonical class. The book is primarily geared towards researchers and graduate students in algebraic geometry who are interested in the structure and classification theory of algebraic varieties. There are, however, presentations of many other applications involving other topics as well - such as Abelian varieties, theta functions and automorphic forms on bounded domains.
Strong Rigidity for Kähler Manifolds and the Construction of Bounded Holomorphic Functions
In this paper we shall give a survey of the known results and methods concerning the strong rigidity of Kahler manifolds and present some new related results. The important phenomenon of strong rigidity was discovered by Professor G.D. Mostow in the case of locally symmetric nonpositively curved Riemannian manifolds. He proved [18] that two compact locally symmetric nonpositively curved Riemannian manifolds are isometric up to normalization constants if they have the same fundamental group and neither one contains a closed one or two dimensional totally geodesic submanifold that is locally a direct factor. This last assumption is clearly necessary because of the existence of non-trivial holomorphic deformations of any compact Riemann surface of genus at least two. Mostow’s result says that if one can rule out the possibility of contribution to the change of metric structure from certain submanifolds of dimension two or lower, the metric structure is rigidly determined by the topology for compact locally symmetric nonpositively curved manifolds. Mostow’s result also holds for the non-compact complete case under the assumption of finite volume.