### 12.5.2022-13.5.2022

**Location:**** GC A1 416 for Thursday, and GR A3 31 for Friday ( click on the number of the room for a map)**

**Schedule:**

**12 May**

14:00-15:00 **Victor do Valle Pretti (Université de Bourgogne)
**

Determinants, even instantons and Bridgeland stability

*Abstract: *

*The moduli of instantons bundles over a Fano threefold have been under investigation by many authors during the last 50 years. Their relation with exceptional collections and monads is already proven to be useful in many situations, such as the ADHM equations and D.Faenzi’s work, for example. During this talk, we will see how to prove they’re stable in the sense of Bridgeland and obtain their moduli space as an open subset of the moduli of Bridgeland stable objects. This is done by finding a region where Bridgeland and quiver stability coincide, hence also obtaining general information about these moduli spaces. The general theory behind this association was proven by E.Macr\`{i}, here we provide a systematic way of describing the quiver regions for any smooth projective variety, if they exist.*

15:00-15:30 coffee

15:30-16:30 **Pascal Fong (Universität Basel)
**

*Title: *Algebraic subgroups of the group of birational transformations of ruled surfaces

*Abstract: We classify the maximal algebraic subgroups of $\mathrm{Bir}(C\times \mathbb{P}^1)$, when $C$ is a smooth curve of positive genus.
*

16:30-17:00 coffee

17:00-18:00 **Luca Tasin (Università di Milano)
**

Higher dimensional slope inequalities

*Abstract: *

*Consider a family of varieties f: X-> T, where T is a curve. We prove several inequalities about the slope of f, which are generalisations of the classical Xiao and Cornalba-Harris inequalities in the case where X is a surface. We then apply our results to the KSB moduli space of stable varieties to study the ample cone of such spaces. The talk is based on a joint work with Giulio Codogni and Filippo Viviani.*

18:30 Dinner

**13 May**

9:00-9:15 coffee

9:15-10:15 **Pierre-Alexandre Gillard (Université de Bourgogne)
**

Torus actions on affine varieties over characteristic zero fields

*Abstract:*

*Torus actions on affine varieties over algebraically closed fields of characteristic zero were described by Altmann and Hausen in 2006 in terms of polyhedral divisors on a certain rational quotient for the torus action.*

*Using Galois descent tools, we will explain how to extend the Altmann-Hausen description over arbitrary fields of characteristic zero. Then, we will discuss certain particular cases in more details using birational geometry tools.*

* *10:15-10:30 coffee

10:30-11:30 **Quentin Posva (EPFL)**

Stable surfaces in positive characteristic and their moduli

*Abstract:
*

The theory of KSBA stable varieties in characteristic zero is nowadays well-understood: we have a good grasp on their moduli and on their deformations. In positive characteristic, many questions are still open: in particular, there is no proof yet of the properness of the moduli space. In this talk, I will report on some recent results about stable surfaces in positive characteristic, focusing on the technical tools that are necessary to study the non-normal ones.

11:30-11:45 coffee

11:45-12:45 **Jarod Alper (University of Washington, Seattle)
**

Coherent completeness in positive characteristic

*Abstract: *

Formal GAGA is a fundamental result asserting that a coherent sheaf on a scheme proper over a complete local noetherian ring is the same as a compatible system of coherent sheaves on the thickenings of its central fiber. We will discuss generalizations of this result to algebraic stacks and explain how such results can be used to prove local structure theorems for algebraic stacks. After reviewing joint work with Hall and Rydh which establishes a satisfactory result in characteristic 0, we will discuss partial progress in joint work with Hall and Lim on extending this result to positive characteristic.

### 7.11.2019-8.11.2019

**Location:**** CH B3 31 for all talks except the last one, which is in MA A3 30 ( click on the number of the room for a map)**

**Schedule:**

**7 Nov**

14:00-15:00 **Calum Spicer**

*Tilte:* Mori theory and foliations

*Abstract: *We will explain some of the ideas behind the study of the birational geometry of foliations, as well as indicating some recent progress in the case of codimension one foliations on threefolds. Features joint work with P. Cascini and R. Svaldi.

15:00-15:30 coffee

15:30-16:30 **Emelie Arvidsson**

*Title: *Kodaira type vanishing theorems on log del Pezzo surfaces of fixed index in positive characteristic

*Abstract: *Abstract: I will discuss Kodaira vanishing theorem on a log del Pezzo surface of fixed Gorenstien index over an algebraically closed field of positive characteristic. By a result of of Cascini, Tanaka and Witaszek Kodaira vanishing holds on such surfaces provided that the characteristic of the base field is sufficiently large. I will present a bound on the characteristic of the base field for which this vanishing may fail in terms of the index. As a consequence, we show that Kodaira vanishing theorem holds on a Gorenstein del Pezzo surface over an algebraically closed field of characteristic greater than or equal to 9221.

16:30-17:00 coffee

17:00-18:00 **Egor Yasinsky**

*Title: *Birational automorphisms of del Pezzo fibrations

*Abstract: *We study groups of birational automorphisms of 3-dimensional del Pezzo fibrations using the Sarkisov program. After explaining main ideas, I will focus on the case of cubic del Pezzo fibrations X. We shall construct some non-trivial homomorphism from Bir(X) to a free product of infinitely many groups of order 2. As an application, we will see that in the Cremona group of rank 3, all connected algebraic subgroups generate a proper normal subgroup. Joint work with J. Blanc.

19:00 Dinner

**8 Nov**

9:00-9:15 coffee

9:15-10:15 **Roberto Diaz**

*Title:* Action of the additive group on affine ind-varieties

*Abstract: *Let V be an affine algebraic variety on C and O(V ) its ring of regular functions, a known result is the correspondence between actions of the additive group Ga = (C, +) on V and locally nilpotent derivations on O(V ). In this talk I will describe a generalization of this correspondence to the category of affine ind-varieties.

10:15-10:30 coffee

10:30-11:30 **Stéphane Druel**

*Title: *On foliations with semi-positive anti-canonical bundle

*Abstract: *I will discuss the structure of (regular) foliations with semi-positive anti-canonical bundle on complex projective manifolds. I will also describe the (mostly conjectural) structure of foliations with numerically trivial canonical class.

11:30-11:45 coffee

11:45-12:45 **Ulrike Riess**

*Title: *Base loci of big and nef line bundles on irreducible symplectic varieties

*Abstract: *In the first part of this talk, I give a complete description of the divisorial part of the base locus of big and nef line bundles on irreducible symplectic varieties (under certain conditions). This is a generalization of well-known results of Mayer and Saint-Donat for K3 surfaces. In the second part, I will present what is currently known on the non-divisorial part.

12:45- 14:15 Lunch

14:15-15:15 **Yohan Brunebarbe**

*Title: *Algebraicity of period maps via o-minimal geometry

*Abstract: *In this talk I will introduce o-minimal geometry and illustrate its relevance to proving algebraicity of certain analytically defined objects. As an application, I will explain that the period maps associated to variations of pure Hodge structures are algebraic in corestriction to their image, as conjectured by Griffiths. This is joint work with Benjamin Bakker and Jacob Tsimerman.

### 3.5.2018-4.5.2018

**Location:**3 May : GR B3 30, map

4 May: MA A3 31, map

**Schedule:**

**3 May**

14:00-15:00 **Adrien Dubouloz**

*Tilte:* Cylinders in Mori Fiber Spaces

*Abstract:* A cylinder in an algebraic variety is a Zariski open subset isomorphic to the product of a variety with the affine line. Every smooth projective variety containing such a cylinder has a birational model which is a Mori Fiber Space over a base. Cylinders in Fano varieties of dimension 3 and 4 have received quite a lot of attention recently in connection to the existence of additive group actions on certain of their affine cones. In this talk, I will focus on the question of existence of such cylinders in total spaces of certain strict Mori Fiber Spaces: del Pezzo fibrations and MFS of relative dimension 3 whose general fibers are isomorphic to the quintic del Pezzo threefold V5. (Joint work with T. Kishimoto, Saitama University).

15:00-15:30 coffee

15:30-16:30 **Nicholas Shepherd-Barron**

*Title: *del Pezzo surfaces and effective Torelli in genus three

*Abstract: *The tropes and singularities of a Kummer surface determine a curve of genus two in a straightforward way. In this talk we describe a similar picture in genus three. This is joint work with M. Fryers.

16:30-17:00 coffee

17:00-18:00 **Giulio Codogni**

*Title:* Positivity of the Chow-Mumford line bundle for families of K-stable klt Fano varieties

*Abstract:* The Chow-Mumford (CM) line bundle is a functorial line bundle defined on the base of any family of polarized varieties, in particular on the base of families of klt Fano varieties. It is conjectured that it yields a polarization on the conjectured moduli space of K-semi-stable klt Fano varieties. This boils down to showing semi-positivity/positivity statements about the CM-line bundle for families with K-semi-stable/K-polystable fibers.

In this talk, I will present a proof of the necessary semi-positivity statements in the K-semi-stable situation, and the necessary positivity statements in the uniform K-stable situation, including in both cases variants assuming stability only for very general fibers. These results work in the most general singular situation (klt singularities), and the proofs are algebraic, except the computation of the limit of a sequence of real numbers via the central limit theorem of probability theory. I will also present an application to the classification of Fano varieties. This is a joint work with Zs. Patakfalvi.

19:00 Dinner for the participants in the Brasserie Lausanne-Moudon

**4 May**

8:45-9:00 coffee

9:00-10:00 **Immanuel van Santen**

*Title:* Embeddings of lines and planes into algebraic groups

*Abstract:* In this talk we will study closed embeddings of affine varieties into affine algebraic groups G up to algebraic automorphisms of the underlying variety of G. After recalling some classical results concerning embeddings into the affine space, we will focus on embeddings of the affine line into arbitrary groups and embeddings of the affine plane into SL_2. Mainly we will discuss the following two results:

-If G is a group without characters of dimension different than

three, then all embeddings of the affine line are equivalent.

-There is an infinite family of pairwise non-equivalent

embeddings of the affine plane into SL_2.

This is joint work with Peter Feller and Jérémy Blanc.

10:00-10:30 coffee

10:30-11:30 **Zhiyu Tian**

*Title:* Motivic crepant resolution conjecture and Chow ring of hyperKähler varieties.

*Abstract:* I will explain a joint program with Lie Fu, where we consider a motivic version of the crepant resolution conjecture in Gromov-Witten theory. I will also discuss an application to the study of Chow rings of Hilbert schemes of points on K3 surfaces.

11:30-11:45 coffee

11:45-12:45 **Jean Fasel**

*Title:* Motivic triviality of the Koras-Russell threefolds of the first kind

*Abstract:* In this talk, we will try to axiomatize the motivic homotopy category of Morel-Voevodsky using the notion of pretriangulated category.

The advantage of this approach is to allow researchers not familiar with motives to work nevertheless in this framework.

As an illustration of the axiomatization, we will show that the Koras-Russell threefolds of the first kind are motivically trivial. This is a joint work with A. Dubouloz.”

12:45- 14:15 Lunch for the paricipants in “Le Parmentier”

14:15-15:15 **Filippo Viviani**

*Title:* On the cone of effective cycles on the symmetric products of curves

*Abstract:* I will report on a joint work with F. Bastianelli, A. Kouvidakis and A. F. Lopez in which we study the cone of (pseudo-)effective cycles on symmetric products of a curve.

We first prove that the diagonal cycles span a face of the pseudo-effective cone of cycles in any given dimension. Secondly, we look at the contractibility faces associated to the Abel-Jacobi morphism towards the Jacobian and in many cases we are able to compute their dimensions.

### 20.10.2017

Location: EPFL, PH H3 33

Schedule:

10:00-10:15 coffee (in front of the lecture room).

10:15-11:15 **Victor Lozovanu**: Convex geometry and positivity aspects in algebraic geometry

Abstract: Intersection numbers are probably the most important tool of studying algebraic varieties. They satisfy certain convexity/continuity properties and in some cases have very geometric description. In early 2000 Okounkov showed how to associate convex sets to a divisor, so that intersection numbers appear naturally as euclidean volumes. This gives a very natural explanation of the convexity nature of intersection numbers. More importantly it opens the door to studying algebraic varieties through convex shapes. In this talk I will try to explain some of these ideas and hopefully give some insight to some interesting applications to questions about local/global geometry of algebraic varieties.

11:15-11:30 coffee break

11:30-12:30 **Alex Küronya****:** *Functions on Newton-Okounkov bodies*

Abstract: Newton-Okounkov bodies are a collection of convex bodies

associated to divisors, or, more generally, graded linear series, that

can be thought of as convex geometric models of the corresponding

algebro-geometric objects. In this talk we take the modelling process

one step further and study concave functions on Newton-Okounkov bodies

that arise from filtrations on the section ring of the underlying line

bundle. We discuss both the formal aspects as well as some applications.

12:45-14:15 Lunch

14:30-15:30 **Anne Lonjou**** : **Cremona group and hyperbolic spaces

Abstract: The Cremona group is the group of birational

transformations of the projective plane. It acts on a hyperbolic space

which is an infinite dimensional version of the hyperboloid model of

H^n. This action is the main recent tool to study the Cremona group.

After defining it, we will study its Voronoï tesselation, and describe

some graphs naturally associated with this construction. Finally we will

discuss which of these graphs are Gromov-hyperbolic.

### 12.05.2017

Location: EPFL, GC A1 416

Schedule:

9:30-10:00 coffee (in front of the lecture room).

10:00-11:00 **Joe Waldron**: General fibres of Mori fibre spaces in positive characteristic

Abstract: A morphism between smooth varieities in characteristic zero has smooth general fibre, but this fails badly in positive characteristic (e.g. for quasi-elliptic surfaces). We use the dictionary between purely inseparable morphisms and foliations of the tangent bundle to put restrictions on this failure. One consequence is that generic smoothness holds for terminal Mori fibre contractions of 3 folds in characteristic p at least 11. This is work in progress with Zsolt Patakfalvi.

11:00-11:30 coffee break

11:30-12:30 **Vladimir Lazić**: Two conjectures in birational geometry

Abstract: I will discuss recent progress on two conjectures in birational geometry: the nonvanishing conjecture and a conjecture of Mumford. This is joint work with Thomas Peternell.

12:45-14:15 Lunch

14:30-15:30 **Mattias Hemmig: **The complement problem for the affine plane

Abstract: We consider complements of irreducible closed curves in the affine plane. Given an isomorphism between two such complements, does it follow that the curves are isomorphic? This question was posed by Hanspeter Kraft in 1995. We give a negative answer by constructing explicit counterexamples, over an arbitrary base field. Nevertheless, counterexamples are quite exceptional – for instance both curves are necessarily isomorphic to open subsets of the affine line. This is a joint work with Jérémy Blanc and Jean-Philippe Furter.