J. Murre has conjectured that every smooth projective variety X of dimension d admits a decomposition of the diagonal δ=p$_0$+…+p$_2d$ ∈ CH$^d$(X × X) [otimes ] Q such that the cycles p$_i$ are orthogonal projectors which lift the Künneth components of the identity map in étale cohomology. If this decomposition induces an intrinsic filtration on the Chow groups of X, we call it a Murre decomposition. In this paper we propose candidates for such projectors on 3-folds by using fiber structures. Using Mori theory, we prove that every smooth uniruled complex 3-fold admits a Murre decomposition.

In this article we prove a Chern–Lashof inequality for immersions of manifolds with H-spherical ends. Related to this inequality we discuss different types of tightness. In particular we shall prove that an immersion of a manifold with at least two H-spherical ends is tightly immersed only if the ends are of a certain geometric type (Wintgen immersion).

In this paper we prove that an isometric stable minimal immersion of a complete oriented surface into a hyperkähler 4-manifold is holomorphic with respect to an orthogonal complex structure, if it satisfies a Bernstein-type assumption on the Gauss-lift. This result generalizes a theorem of Micallef for minimal surfaces in the euclidean 4-space. An example found by Atiyah and Hitchin shows that the assumption on the Gauss-lift is necessary.

We prove a lower bound for the number of singular fibres of a non-isotrivial semi-stable family of curves over the projective line, in positive characteristic. The result is the same obtained previously by Beauville in characteristic zero, namely that the number is at least 4. Furthermore some consequences are discussed in the case that the number is exactly 4.

We calculate l-adic nearby cycles in the étale cohomology for families with log smooth reduction using log étale cohomology. In particular, nearby cycles for log smooth families coincide with tame nearby cycles, as L. Illusie expected, and nearby cycles for semistable families depend only on the first infinitesimal neighborhood of the special fiber.

This note investigates compact complex manifolds X of dimension 3 with second Betti number b$_2$(X) = 0. If X admits a non-constant meromorphic function, then we prove that either b$_1$(X) = 1 and b$_3$(X) = 0 or that b$_1$(X) = 0 and b$_3$(X) = 2. The main idea is to show that c$_3$(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 S$^6$ cannot admit a non-constant meromorphic function. Furthermore we investigate the structure of threefolds with b$_2$(X) = 0 and algebraic dimension 1, in the case when the algebraic reduction X → P$_1$ is holomorphic.

The main purpose of this work is to introduce noncommutative relative schemes and establish some of basic properties of schemes and scheme morphisms. In particular, we prove an analogue of the canonical bijection: Hom$_schemes/k$((X, O), Spec(A))[bsime ] Hom$_k-alg$ (A,Γ (X, O)). We define a noncommutative version of the Čech cohomology of an affine cover and show that the Čech cohomology can be used to compute higher direct images. This fact is applied here to compute cohomology of invertible sheaves on skew projective spaces and in [LR3] to study D-modules on quantum flag varieties.