# School - Flows and Limits in Kähler Geometry

### Titles and abstracts

• Bo Berndtsson (Chalmers University) : Direct image bundles and variations of complex structures

Given a smooth proper fibration $p:\mathcal X\to B$ and $L$ a line bundle over $\mathcal X$, the direct image $$E:= p_*(L)$$ is in many cases a holomorphic vector bundle over $B$. Its fibers are the spaces of holomorphic sections of $L$ over the fibers of $p$, $X_t=p^{-1}(t)$, and they can be given various $L^2$-metrics. In case the fibration is of relative dimension $n$ so that the fibers are compact Riemann surfaces, special cases of this situation can be used to study the variation of complex structures on the fibers $X_t$. (The fibers are all diffeomorphic, but their complex structure varies with $t$, so we can view the family $X_t$ as a family of variations of complex structures on one fixed smooth manifold.) When the relative dimension is higher than one the situation is more complicated and one needs to consider also higher direct images. I will discuss the problems that arise in this connection, with previous work of Siu, Schumacher and To-Yeung and some recent joint work with Xu Wang and Mihai Paun.

• Hans-Joachim Hein (Fordham University) : Tangent cones of Calabi-Yau varieties

It has been known for about 10 years that the classical Calabi-Yau theorem on the existence and uniqueness of Ricci-flat Kahler metrics on smooth complex manifolds with zero first Chern class can be extended to a natural setting of weak Kahler metrics on singular complex varieties. However, until relatively recently nothing was known - even in the simplest nontrivial examples - about the precise asymptotic behavior of these weak Ricci-flat metrics at the singularities of the underlying varieties. I will explain work of Donaldson-Sun, H-Naber and H-Sun that resolves this question in certain cases.

• Valentino Tosatti (Northwestern University) : Metric Limits of Calabi-Yau Manifolds

In this mini-course I will give an introduction to the study of limits of Ricci-flat Kahler metrics on a compact Calabi-Yau manifold when the Kahler class degenerates to the boundary of the Kahler cone. Analytically, the problem is to prove suitable uniform a priori estimates for solutions of a degenerating family complex Monge-Ampère equations, away from some singular set. Geometrically, this can be used to understand the Gromov-Hausdorff limit of these metrics. And if the manifold is projective algebraic and the limiting class is rational, the limits possess an algebraic structure and are obtained from the initial manifold via contraction morphisms from Mori theory.

• Jeff Viaclovsky (Wisconsin University) : The geometry of SFK ALE metrics

I will discuss some of the basics of scalar-flat Kaehler (SFK) metrics, and focus on the geometry of SFK metrics which are asymptotically locally Euclidean (ALE). These space arise as "bubbles" in the compactness theory of Calabi's extremal Kaehler metrics. I will also present some of the deformation theory of SFK ALE metrics.

• Thibaut Delcroix (ENS Paris) : Kähler geometry of horospherical manifolds

Horospherical manifolds form a class of almost homogeneous manifolds whose Kähler geometry is very close to that of toric manifolds. They strictly contain homogeneous toric bundles, to which a lot of results holding for toric manifolds have been extended. I will present horospherical manifolds, trying to convince you that they are not much harder to deal with, and in particular I will present the criterion for K-stability in the Fano case that follows either from my work on spherical varieties, or from a direct, Wang-Zhu type, approach.

• Eleonora Di Nezza (Imperial College) : Monge-Ampère energy and weak geodesic rays

The recent proof of Demailly's conjecture by Witt Nyström gives another evidence that pluripotential theory play a key role when working with complex Monge-Ampère equations in order to solve problems in differential and algebraic geometry. In this talk we investigate pluripotential tools: we characterise Monge-Ampère energy classes in terms of envelopes. And in order to do that, we develop the theory of weak geodesic rays in a big cohomogy class. We also give a positive answer to an open problem in pluripotential theory. This is a joint work with Tamas Darvas and Chinh Lu.

• Jakob Hultgren (Chalmers University) : Coupled Kähler-Einstein Metrics

A central theme in complex geometry is to study various types of canonical metrics, for example Kähler-Einstein metrics and cscK metrics. In this talk we will introduce the notion of coupled Kähler-Einstein (cKE) metrics which are k-tuples of Kähler metrics that satisfy certain coupled Kähler-Einstein equations. We will discuss existence and uniqueness properties and elaborate on related algebraic stability conditions. (Joint work with David Witt Nyström)

• Zakarias Sjostrom Dyrefelt (Université de Toulouse) : K-stability of constant scalar curvature Kähler manifolds

In this talk we introduce a variational/pluripotential approach to the study of K-stability of Kähler manifolds with transcendental cohomology class, extending a classical picture for polarised manifolds. Our approach is based on establishing a formula for the asymptotic slope of the K-energy along certain geodesic rays, from which we deduce that cscK manifolds are K-semistable. Combined with a recent properness result of R. Berman, T. Darvas and C. Lu we further deduce uniform K-stability of cscK manifolds with discrete automorphism group, thus confirming one direction of the YTD conjecture in this setting. If time permits we also discuss possible extensions of these results to the case of compact Kähler manifolds admitting holomorphic vector fields.