Talk

We study Grothendieck's Lefschetz standard conjecture on a smooth complex projective variety. In degree 2, we reduce it to a local statement concerning local deformations of vector bundles on X. When X is hyperkaehler, we give explicit criteria which imply the conjecture, using Verbitsky's theory of deformations of hyperholomorphic bundles.

Let f be a rational function that is the composite of two rational functions g,h. In this talk we discuss the question of what can be said about the composition factors g,h when we assume that f is lacunary e.g. in the sense that the number of terms in a given representation of f as quotient of two polynomials is fixed (but also other notions of lacunarity are of interest and will be studied). The results support and quantify the intuitive expectation that rational operations of large degree tend to destroy the lacunarity.

Thin buildings (in the sense of Tits) arise as coset geometries of Coxeter groups, most of the spherical buildings as coset geometries of groups with a BN-pair. We generalize the notion of a group to ``generalized groups" in such a way that arbitrary (not necessarily thin or spherical) buildings arise as coset geometries of ``generalized Coxeter groups". Each set of right cosets of a subgroup of a given group turns out to be a generalized group. These generalized groups are called ``schurian".

In this talk we investigate properties of the coefficients of mock modular forms. After an appropriate correction term related to the shadow of the mock modular form, one sees interesting congruences related to modular forms. We will show that these congruences arise because a mock modular form combines with the Eichler integral of its shadow to produce a $p$-adic modular form.

Let X be a K3 surface and E an irreducible curve on X. Then the following are well known to be equivalent: 1) E is a smooth rational curve. 2) E has self-intersection -2. 3) E is the exceptional divisor of a birational morphism from X to a normal projective surface Y with an isolated singular point. Furthermore, if e is a cohomology class of Hodge type (1,1) and self-intersection -2, then e=[E] or e=-[E] for an effective divisor E, and E becomes irreducible, under a generic small deformation of the pair (X,e).

Brieskorn varietes admit $S^1$-actions whose isotropy groups are finite. Choose a Brieskorn variety diffeomorphic to a sphere and an odd prime p which is relatively prime to the order of all isotropy groups. Then the orbit space of the induced $Z/p$-action is a homotopy lens space -- a closed manifold homotopy equivalent to a lens space. A homotopy lens space is fake if it is not homeomorphic to a classical lens space. Using Reidemeister torsion, we show some of these homotopy lens spaces are fake lens spaces.