**Topics around algebraic and symplectic geometry as well as mirror symmetry**

This colloquium invites mostly external speakers. It is funded by the Emmy Noether grant "Degenerations of Calabi-Yau Manifolds and Related Geometries" of Helge Ruddat.

**We meet during the semester from 13.00 - 13:50 in Hilbertraum 05-432 (JGU Mainz, Mathematisches Institut, Staudingerweg 9, 55099 Mainz).**

**Programm (please find the abstracts below)**

18.10.2017 17:00 Robin Guilbot

*On embedded Mirror Symmetry*

18.10.2017 09:30 Valentin Tonita

*K-theoretic mirror formulae*

16.10.2017 10:00 Matej Filip

*Hochschild cohomology and Deformation quantization of affine toric varieties*

27.06.2017 12:00 Martin Ulirsch (University of Michigan, USA)

*A moduli stack of tropical curves*

02.05.2017 12:00 Michel van Garrel (Universität Hamburg)

*Rational curves in log K3 surfaces*

03.11.2016 13:00 Tom Sutherland (Università degli Studi di Pavia)

*Stability conditions from periods of elliptic curves*

19.07.2016 13:00 Tim Kirschner (Universität Duisburg-Essen)

*Finite quotients of three-dimensional complex tori*

14.07.2016 14:00 James Pascaleff (University of Illinois)

*Symplectic Cohomology and Wall Crossing*

26.04.2016 13:00 Mohammad Akhtar (Imperial College London)

* Mirror symmetry and the classification of Fano varieties*

07.01.2016 12:00 Andreas Gross (Universität Kaiserslautern)

*Intersection Theory on Tropicalizations of Toroidal Embeddings*

17.12.2015 12:00 Sara Filippini (Universität Zürich)

*Refined curve counting and the tropical vertex group*

29.10.2015 12:00 Aleksey Zinger (MPIM Bonn)

*Normal Crossings Divisors for Symplectic Topology*

25.06.2015 12:00 Peter Overholser (Leuven/Imperial College London)

*Descendent tropical mirror symmetry for P2*

## Abstracts

- 18.10.2017 17:00 Robin Guilbot: On embedded Mirror Symmetry
- 18.10.2017 09:30 Valentin Tonita: K-theoretic mirror formulae
- 16.10.2017 10:00 Matej Filip: Hochschild cohomology and deformation quantization of affine toric varieties
- 27.06.2017 12:00 Martin Ulirsch: A moduli stack of tropical curves
- 02.05.2017 12:00 Michel van Garrel: Rational curves in log K3 surfaces
- 03.11.2016 13:00 Tom Sutherland: Stability conditions from periods of elliptic curves
- 19.07.2016 13:00 Tim Kirschner: Finite quotients of three-dimensional complex tori
- 14.07.2016 14:00 James Pascaleff: Symplectic Cohomology and Wall Crossing
- 26.04.2016 13:00 Mohammad Akhtar: Mirror symmetry and the classification of Fano varieties
- 07.01.2016 12:00 Andreas Gross: Intersection Theory on Tropicalizations of h3 Embeddings
- 17.12.2015 12:00 Sara Filippini: Refined curve counting and the tropical vertex group
- 29.10.2015 12:00 Aleksey Zinger: Normal Crossings Divisors for Symplectic Topology
- 25.06.2015 Peter Overholser: Descendent tropical mirror symmetry for P2

### 18.10.2017 17:00 Robin Guilbot: On embedded Mirror Symmetry

A wide majority of the known instances of Mirror Symmetry between families of Calabi-Yau varieties are realized as complete intersections in toric varieties.

In these examples the features of Mirror Symmetry are more or less direct consequences of convex-combinatorial dualities. But the elegance of these constructions

is somehow balanced by their peculiarity : toric complete intersections are expected to form a small minority of all the Calabi-Yau varieties.

I will Review the most famous toric mirror constructions, describe a generalization of the hypersurface case introduced in joint work with M. Artebani and P. Comparin,

and sketch the foundations of a new construction for non-complete intersections based on embedded toric degenerations, following J. Böhm’s PhD thesis.

### 18.10.2017 09:30 Valentin Tonita: K-theoretic mirror formulae

Permutation equivariant K-theoretic Gromov Witten invariants, introduced by Givental, are

certain Euler characteristics on the moduli spaces of stable maps to a (smooth, projective)

variety X. I will define the invariants and show how to write K-theoretic I-functions for large classes of varieties

(e.g. toric, certain complete intersections), i.e. certain q-hypergeometric series which are

generating series of these invariants in genus zero . Time permitting, I will discuss the ideas behind

the proofs of these results.

### 16.10.2017 10:00 Matej Filip: Hochschild cohomology and deformation quantization of affine toric varieties

For an affine toric variety we give a convex geometric description of the Hodge decomposition of its Hochschild cohomology. Using this description we prove that every Poisson structure on a possibly singular affine toric variety can be quantized in the sense of deformation quantization. Restricting to the commutative case, recent developments in constructing the versal deformation of an affine toric variety will be explained.

### 27.06.2017 12:00 Martin Ulirsch: A moduli stack of tropical curves

In this talk I am going to give an introduction to these fascinating moduli spaces and discuss recent work with Renzo Cavalieri, Melody Chan, and Jonathan Wise (arXiv 1704.03806), where we propose the notion of a moduli stack of tropical curves as a geometric stack over the category of rational polyhedral cones. Using this 2-categorical framework one can give a natural interpretation of the forgetful morphism as a universal curve. Moreover, I will propose a way of describing the process of tropicalization via logarithmic geometry in the sense of Kato-Illusie using the theory of Artin fans. Finally, given time, I will also report on an ongoing follow-up project (joint with Margarida Melo, Filippo Viviani, and Jonathan Wise) that uses these techniques to construct the universal Picard variety in logarithmic and tropical geometry.

### 02.05.2017 12:00 Michel van Garrel: Rational curves in log K3 surfaces

In this talk, we address the basic question of how to count rational curves in log K3 surfaces. We will present partial results in that direction and give a full conjectural description. This is baded on two joints works, one with T. Graber and H. Ruddat, and the other one with J. Choi, N. Takahashi ad S. Katz.

### 03.11.2016 13:00 Tom Sutherland: Stability conditions from periods of elliptic curves

I will describe spaces of stability conditions on some Calabi-Yau-3 categories with a simple combinatorial presentation through the study of the period map of meromorphic differentials on associated families of elliptic curves.

### 19.07.2016 13:00 Tim Kirschner: Finite quotients of three-dimensional complex tori

I will report on a current project with Patrick Graf (Bayreuth). Using Graf's recent results about the algebraic approximation of Kähler threefolds of Kodaira dimension zero, we show that a three-dimensional compact, connected Kähler space *X* with isolated canonical singularities is the finite quotient of a complex torus if and only if the first and second Chern classes of *X* vanish. This brings together an old theorem of Yau (where *X* is smooth) and a theorem of Shepherd-Barron and Wilson (where *X* is projective).

### 14.07.2016 14:00 James Pascaleff: Symplectic Cohomology and Wall Crossing

In this talk I will describe a way that certain wall-crossing formulae can be seen in terms of symplectic cohomology, which is a Floer theoretic invariant of non-compact symplectic manifolds. In the case of log Calabi-Yau manifolds, this invariant is supposed to be mirror-dual to the poly-vector fields. I will draw connections to the theory of cluster varieties as studied by Gross-Hacking-Keel-Kontsevich. This is partially based on discussions with Dmitry Tonkonog (Cambridge).

### 26.04.2016 13:00 Mohammad Akhtar: Mirror symmetry and the classification of Fano varieties

The classification of Fano varieties is an important long-standing problem in algebraic geometry. A new approach to this problem via mirror symmetry was recently proposed by Coates-Corti-Galkin-Golyshev-Kasprzyk. Their philosophy was that Fano varieties can be classified by studying their Laurent polynomial mirrors. This talk will survey the results of a collaborative effort to apply this philosophy to the classification of Fano orbifold surfaces. We will describe a conjectural picture which suggests that classifying suitable deformation classes of certain Fano orbifold surfaces is equivalent to classifying Fano lattice polygons up to an appropriate notion of equivalence. Central to this framework is the notion of mirror duality (between a Fano orbifold surface and a Laurent polynomial) and the closely related operations of algebraic and combinatorial mutations. We will also discuss how combinatorial mutations allow us to find mirror dual Laurent polynomials in practice and will give experimental evidence supporting our conjectures.

### 07.01.2016 12:00 Andreas Gross: Intersection Theory on Tropicalizations of h3 Embeddings

A central goal of tropical geometry is to give combinatoric descriptions of algebro-geometric objects. In enumerative geometry, these description ideally give rise to so-called correspondence theorems, which state that some given algebraic enumerative problem can be translated into a tropical enumerative problem with the same solution. The tropical intersection theory of Allermann and Rau has become a useful tool in tropical enumerative geometry, its connection to algebraic geometry being based on the description of the intersection ring of complete toric varieties by Fulton and Sturmfels. Unfortunately, moduli spaces are rarely toric, yet in many cases they are toroidal.

In my talk I will outline how to extend the scope of tropical intersection theory to be able to describe certain intersections on toroidal varieties.

### 17.12.2015 12:00 Sara Filippini: Refined curve counting and the tropical vertex group

The tropical vertex group of Kontsevich and Soibelman is generated by formal symplectomorphisms of the 2-dimensional algebraic torus. It plays a role in many problems in algebraic geometry and mathematical physics. Based on the tropical vertex group, Gross, Pandharipande and Siebert introduced an interesting Gromov-Witten theory on weighted projective planes which admits a very special expansion in terms of tropical counts.

I will describe a refinement or "q-deformation" of this expansion, motivated by wall-crossing ideas, using Block-Goettsche invariants. This leads naturally to the definition of a class of putative q-deformed curve counts. We prove that this coincides with another natural q-deformation, provided by a result of Reineke and Weist in the context of quiver representations, when the latter is well defined. This is joint work with Jacopo Stoppa.

### 29.10.2015 12:00 Aleksey Zinger: Normal Crossings Divisors for Symplectic Topology

I will describe purely symplectic notions of normal crossings divisor and configuration. They are compatible with the existence of the desired auxiliary almost Kahler structures, provided ``existence" is suitably interpreted. These notions lead to a multifold version of Gompf's symplectic sum construction. They also imply that Brett Parker's work on exploded manifolds concerns a multifold version of the usual symplectic sum (or degeneration) formula for Gromov-Witten invariants. We hope our approach can be extended to more general singularities and provide purely symplectic analogues of the singularities and their deformations appearing in the Gross-Siebert program. This is joint work with Mark McLean and Mohammad Tehrani.

### 25.06.2015 Peter Overholser: Descendent tropical mirror symmetry for P2

The Gross-Siebert program can be seen as an attempt to understand mirror symmetry from a tropical perspective. Gross has realized this goal in a particular example, giving a tropical description of mirror symmetry for P2. I will show how his construction can be modified to to yield a novel mirror symmetric relationship.