We give a noncommutative analogue of Castelnuovo's classic theorem that (-1) lines on a smooth surface can be contracted, and show how this may be used to construct an explicit birational map between a noncommutative P^2 and a noncommutative quadric surface. This has applications to the classification of noncommutative projective surfaces, one of the major open problems in noncommutative algebraic geometry. We will not assume a background in noncommutative ring theory. The talk is based on joint work with Rogalski and Staffor

# Past Geometry and Analysis Seminar

An "open de Rham space" refers to a moduli space of meromorphic connections on the projective line with underlying trivial bundle. In the case where the connections have simple poles, it is well-known that these spaces exhibit hyperkähler metrics and can be realized as quiver varieties. This story can in fact be extended to the case of higher order poles, at least in the "untwisted" case. The "twisted" spaces, introduced by Bremer and Sage, refer to those which have normal forms diagonalizable only after passing to a ramified cover. These spaces often arise as quotients by unipotent groups and in some low-dimensional examples one finds some well-known hyperkähler manifolds, such as the moduli of magnetic monopoles. This is a report on ongoing work with Tamás Hausel and Dimitri Wyss.

Sasakian manifolds are odd-dimensional counterparts of Kahler manifolds in even dimensions,

with K-contact manifolds corresponding to symplectic manifolds. It is an interesting problem to find

obstructions for a closed manifold to admit such types of structures and in particular, to construct

K-contact manifolds which do not admit Sasakian structures. In the simply-connected case, the

hardest dimension is 5, where Kollar has found subtle obstructions to the existence of Sasakian

structures, associated to the theory of algebraic surfaces.

In this talk, we develop methods to distinguish K-contact manifolds from Sasakian ones in

dimension 5. In particular, we find the first example of a closed 5-manifold with first Betti number 0 which is K-contact but which carries no semi-regular Sasakian structure.

(Joint work with J.A. Rojo and A. Tralle).

Work of Schoen--Yau in the 70's/80's shows that area-minimizing (actually stable) two-sided surfaces in three-manifolds of non-negative scalar curvature are of a special topological type: a sphere, torus, plane or cylinder. The torus and cylinder cases are "borderline" for this estimate. It was shown by Cai--Galloway in the late 80's that the torus can only occur in a very special ambient three manifold. We complete the story by showing that a similar result holds for the cylinder. The talk should be accessible to those with a basic knowledge of curvature in Riemannian geometry.

After reviewing the main results relating holomorphic Poisson geometry to generalized Kahler structures, I will explain some recent progress in deforming generalized Kahler structures. I will also describe a new way to view generalized kahler geometry purely in terms of Poisson structures.

In the same way that the classical Torelli theorem determines a curve from its polarized Jacobian we show that moduli spaces of parabolic bundles and parabolic Higgs bundles over a compact Riemann surface X also determine X. We make use of a theorem of Hurtubise on the geometry of algebraic completely integrable systems in the course of the proof. This is a joint work with I. Biswas and T. Gómez