# 1999 seminars

## 19/02/2013, Tuesday, 16:30–17:30

Jacopo Stoppa, Università di Pavia
Refined curve counting, quivers, and wall-crossing

I will sketch some aspects of an interesting Gromov-Witten theory on weighted projective planes introduced by Gross, Pandharipande and Siebert. It admits a very special expansion in terms of tropical counts (called the tropical vertex), as well as a conjectural BPS structure. Then I will describe a refinement or "$q$-deformation" of the expansion using Block-Goettsche invariants, motivated by wall-crossing ideas. This leads naturally to a definition of a class of putative $q$-deformed BPS 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 (joint with S. A. Filippini).

## 21/02/2013, Thursday, 16:30–17:30

I will discuss a generalization of the classical Clifford's theorem to singular curves, reducible or non reduced. I will prove that for 2-connected curves a Clifford-type inequality holds for a vast set of torsion free rank one sheaves. I intend to show that our assumptions on the sheaves are the most natural when working with this kind of results. I will moreover show that this result has many applications to the study of the canonical morphism of a singular curve, in particular that it implies a generalization of the classical Noether's theorem to 3-connected curves. This is a joint work with M. Franciosi.

## 12/03/2013, Tuesday, 16:30–17:30

Gromov width of a symplectic manifold $M$ is a supremum of capacities of balls that can be symplectically embedded into $M$. The definition was motivated by the Gromov's Non-Squeezing Theorem which says that maps preserving symplectic structure form a proper subset of volume preserving maps.

Let $G$ be a compact connected Lie group, $T$ its maximal torus, and $\lambda$ be a point in the chosen positive Weyl chamber.

The group $G$ acts on the dual of its Lie algebra by coadjoint action. The coadjoint orbit, $M$, through $\lambda$ is canonically a symplectic manifold. Therefore we can ask the question of its Gromov width.

In many known cases the width is exactly the minimum over the set of positive results of pairing $\lambda$ with coroots of $G$:

$\mathrm{min}\left\{⟨{\alpha }_{j}^{\vee },\lambda ⟩;{\alpha }_{j}\text{a coroot},⟨{\alpha }_{j}^{\vee },\lambda ⟩>0\right\}.$

For example, this result holds if $G$ is the unitary group and $M$ is a complex Grassmannian or a complete flag manifold satisfying some additional integrality conditions.

We use the torus action coming from the Gelfand-Tsetlin system to construct symplectic embeddings of balls. In this way we prove that the above formula gives the lower bound for Gromov width of all $U\left(n\right)$ and most of $\mathrm{SO}\left(n\right)$ coadjoint orbits.

In the talk I will describe the Gelfand-Tsetlin system and concentrate mostly on the case of regular $$U(n)$$ orbits.

## 26/03/2013, Tuesday, 16:30–17:30

Leonardo Macarini, Universidade Federal do Rio de Janeiro
Two periodic orbits on the standard three-sphere

We prove that every contact form on the tight three-sphere has at least two geometrically distinct periodic orbits. This result was obtained recently by Cristofaro-Gardiner and Hutchings using embedded contact homology but our approach instead is based on cylindrical contact homology. An essential ingredient in the proof is the notion of a symplectically degenerate maximum for Reeb flows whose existence implies infinitely many prime periodic orbits (in any dimension). This is joint work with V. Ginzburg, D. Hein and U. Hryniewicz.

## 16/05/2013, Thursday, 16:30–17:30

Matias del Hoyo
On the linearization of certain smooth structures

The linearization theorem for proper Lie groupoids, whose prove was completed by Zung a few years ago, generalizes various results such as Ehresmann theorem for submersions, Reeb stability for foliations, and the Tube Theorem for proper actions. In a work in progress with R. Fernandes we show that this linearization can be achieved by means of the exponential flow of certain metrics, providing both a stronger theorem and a simpler proof. In this talk I will recall the classic linearization theorems, discuss its groupoid formulation, and present our work on riemannian structures for Lie groupoids.

## 21/05/2013, Tuesday, 16:30–17:30

Carlos Florentino, Instituto Superior Técnico
Irreducibility of character varieties of abelian groups

The description of the space of commuting elements in a compact Lie group is an interesting algebro-geometric problem with applications in Mathematical Physics, notably in Supersymmetric Yang Mills theories.

When the Lie group is complex reductive, this space is the character variety of a free abelian group. Let $$K$$ be a compact Lie group (not necessarily connected) and $$G$$ be its complexiﬁcation. We consider, more generally, an arbitrary finitely generated abelian group $$A$$, and show that the conjugation orbit space $\mathrm{Hom}\left(A,K\right)/K$ is a strong deformation retract of the character variety $\mathrm{Hom}\left(A,G\right)/G$.

As a Corollary, in the case when $$G$$ is connected and semisimple, we obtain necessary and sufficient conditions for $\mathrm{Hom}\left(A,G\right)/G$ to be irreducible. This is also related to an interesting open problem about irreducibility of the variety of $$k$$ tuples of $$n$$ by $$n$$ commuting matrices.

## 07/06/2013, Friday, 16:30–17:30

Dennis The, Australian National University
The gap phenomenon in parabolic geometries

Many geometric structures (such as Riemannian, conformal, CR, projective, systems of ODE, and various types of generic distributions) admit an equivalent description as Cartan geometries. For Cartan geometries of a given type, the maximal amount of symmetry is realized by the flat model. However, if the geometry is not (locally) flat, how much symmetry can it have? Understanding this "gap" between maximal and submaximal symmetry in the case of parabolic geometries is the subject of this talk. I'll describe how a combination of Tanaka theory, Kostant's version of the Bott-Borel-Weil theorem, and a new Dynkin diagram recipe led to a complete classification of the submaximal symmetry dimensions in all parabolic geometries of type $(G,P)$, where $G$ is a complex or split-real simple Lie group and $P$ is a parabolic subgroup. (Joint work with Boris Kruglikov.)

## 20/06/2013, Thursday, 16:30–17:30

Manuel Araújo, Instituto Superior Tecnico
Symplectic embeddings into $$\mathbb{C}P^\infty$$

Let $(M,\omega)$ be a compact symplectic manifold with $[\omega]$ integral. It is a Theorem of Gromov (1970) and Tischler (1977) that $(M,\omega)$ symplectically embeds into $\mathbb{C}P^n$ with the Fubini-Study symplectic form, for $n$ large enough. Let $\beta_1(M)$ be the first Betti number of $M$. We refine this result of Gromov and Tischler by showing that the weak homotopy type of the space of symplectic embeddings of such a symplectic manifold into $\mathbb{C}P^\infty$ is $(S^1)^{\beta_1(M)}\times\mathbb{C}P^\infty$.

## 01/07/2013, Monday, 16:30–17:30

Nick Sheridan, Institute for Advanced Study and Princeton University
Homological mirror symmetry

Mirror symmetry is a conjectural relationship between complex and symplectic geometry, and was first noticed by string theorists. Mathematicians became interested in it when string theorists used it to predict counts of curves on the quintic three-fold (just as there are famously 27 lines on a cubic surface, there are 2875 lines on a quintic three-fold, 609250 conics, and so on). Kontsevich conjectured that mirror symmetry should reflect a deeper equivalence of categories: his celebrated Homological Mirror Symmetry conjecture. Most of the talk will be an overview of mirror symmetry with a focus on the symplectic side, leading up to Kontsevich's conjecture. Finally I will describe a proof of Kontsevich's conjecture for the quintic three-fold, and more generally for a Calabi-Yau hypersurface in projective space of any dimension. If time permits I will draw lots of pictures in the one-dimensional case.

## 03/07/2013, Wednesday, 15:00–16:00

J. C. Naranjo and G. P. Pirola, Univ. Barcelona and Univ. Pavia
Isogenies between Jacobians

In this series of two lectures we will report about the following Theorem: Let $Z$ be a subvariety of codimension $k>0$ of the moduli space ${ℳ}_{g}$ of curves of genus $g$ with $g>3k+4$ and let $\chi :JC\prime \to JC$ an isogeny between two Jacobians, where $C$ is generic in $Z$. Then $C$ and $C\prime$ are isomorphic and $\chi$ is the multiplication by an integer. The statement is also true for $k=1$ and $g\ge 5$. This extends a result by Bardelli and Pirola for the case $Z={ℳ}_{g}$ and $g\ge 4$.

There are two natural approaches to this problem. The first one, which will be the content of the first talk, uses infinitesimal variations of Hodge structures in order to translate the statement into a problem concerning the quadrics containing a canonical curve, probably of independent interest. The second one consists in to degenerate to some special (mainly singular stable) curves. This second approach, much more subtle, works for $g\ge 5$. However it needs a good control of the intersection of the closure of $Z$ with the boundary of $\overline{{ℳ}_{g}}$, and for this we have to restrict to $k=1$. This procedure will be explained in the second lecture.

This is a joint work of the speakers with V. Marcucci.

## 08/07/2013, Monday, 11:30–12:30

David Martínez Torres, Utrecht
Non-contractible loops in the diffeomorphism group of coadjoint orbits

A compact connected semisimple Lie group G acts in a Hamiltonian fashion on its coadjoint orbits, i.e. G maps into the group of Hamiltonian transformations of the coadjoint orbit. McDuff and Tolman showed that the induced map on fundamental groups is injective, answering a question of A. Weinstein. In this talk we will show that this is not quite a symplectic phenomenon, but a topological one, since the (finite) fundamental group of G already injects in the fundamental group of the group of diffeomorphisms. This is joint work with I. Mundet.

## 16/07/2013, Tuesday, 16:30–17:30

Eric Sommers, University of Massachusetts
Properties of some resolutions of Schubert varieties

Schubert varieties are certain closed subvarieties of the flag variety of a complex algebraic group $$G$$. They are indexed by elements of the Weyl group $$W$$ of $$G$$. In general they are singular and some of the structure of their singularities can be understood via the resolutions known as the Bott-Samelson resolutions and the resulting interplay between the intersection cohomology of the Schubert variety and the ordinary cohomology of the fibers of these resolutions.

This talk focuses on joint work with my student, Jennifer Koonz, on some generalizations of the Bott-Samelson resolutions, which provide some new information about the singularities of Schubert varieties. Special cases of these resolutions have already appeared in work by Polo, Wolper, Ryan, and Zelevinsky in the case where $$G$$ is the general linear group $$\operatorname{GL}(n)$$ and $$W$$ is the symmetric group $$S_n$$. I will explain the Bott-Samelson resolutions and these new resolutions in some small examples for the general linear group and perhaps finish with a couple of propositions that are valid for arbitrary $$G$$.

## 17/07/2013, Wednesday, 16:30–17:30

Marta Batoreo, UC Santa Cruz
On hyperbolic points and periodic orbits of symplectomorphisms

In this talk, we discuss a variant of the Conley conjecture which asserts the existence of infinitely many periodic orbits of a symplectomorphism if it has a fixed point which is unnecessary in some sense. More specifically, we discuss a result claiming that, for a certain class of closed monotone symplectic manifolds, any symplectomorphism isotopic to the identity with a hyperbolic fixed point must necessarily have infinitely many periodic orbits as long as the symplectomorphism satisfies some constraints on the flux.

## 18/07/2013, Thursday, 16:30–17:30

Pavel Etingov, Massachusetts Institute of Technology
D-modules on Poisson varieties and Poisson traces.

Let $$V$$ be an affine symplectic algebraic variety over $$\mathbb{C}$$, and $$G$$ a finite group of automorphisms of $$V$$ (for example, $$V$$ is a symplectic vector space, and $$G$$ is a subgroup of $$\mathop{Sp}(V)$$). Let $$A$$ be the algebra of regular functions on $$V/G$$, and $$E$$ be the space of linear functionals on $$A$$ which are invariant under Hamiltonian vector fields on $$V/G$$ (so called Poisson traces). It turns out that $$E$$ is finite dimensional. I will explain how to prove and generalize this statement, using the theory of D-modules, and will also describe some applications to noncommutative algebra. This is joint work with Travis Schedler.

## 23/07/2013, Tuesday, 14:30–15:30

The topology of a toric symplectic manifold can be read directly from its orbit space (a.k.a. moment polytope), and much the same is true of the topological generalizations of toric symplectic manifolds and projective toric varieties: quasitoric manifolds, topological toric manifolds and torus manifolds. An origami manifold is a manifold endowed with a closed 2-form with a very mild degeneracy along a hypersurface, and in the toric case its orbit space is an "origami polytope". In this talk we examine how the topology of a toric origami manifold can be read from its orbit space and how these results hold for the appropriate topological generalization of the class of toric origami manifolds, which includes quasitoric manifolds, and some torus manifolds. These results are from ongoing joint work with Tara Holm.

## 29/10/2013, Tuesday, 16:30–17:30

Gonçalo Oliveira, Imperial College
Monopoles in Higher Dimensions

The Monopole (Bogomolnyi) equations are Geometric PDEs in 3 dimensions. In this talk I shall introduce a generalization of the monopole equations to both Calabi Yau and $$G_2$$ manifolds. I will motivate the possible relations of conjectural enumerative theories arising from "counting" monopoles and calibrated cycles of codimension 3. Then, I plan to state the existence of solutions and sketch how these examples are constructed.

## 31/10/2013, Thursday, 16:30–17:30

Tiago Fonseca, Laboratoire d'Annecy-le-Vieux de Physique Théorique, Université de Savoie
The D-Kadomtsev–Petviashvili and its grassmannian description

The Kadomtsev–Petviashvili (KP) equation is an important equation in fluid dynamics: it describes sea waves. It is an integrable model, meaning that we can find "all" solutions. In fact, the solutions are described by a purely geometric object, the grassmaniann. In the case of the equation D-KP (the orthogonal variation of KP) the solutions are described by isotropic grassmannians. In this talk, I'll explain how from an isotropic grassmannian we can build a solution for this equation.

## 19/11/2013, Tuesday, 16:30–17:30

Alfonso Zamora, Instituto Superior Técnico
GIT characterizations of Harder-Narasimhan filtrations

In a moduli space, usually, we impose a notion of stability for the objects and, when constructing the moduli space by using Geometric Invariant Theory, another notion of GIT stability appears, showing during the construction of the moduli that both notions do coincide at the stable and semistable level. For an object which is unstable (this is, contradicting the stability condition) there exists a canonical unique filtration, called the Harder-Narasimhan filtration. Onthe other habd, GIT stability is checked by 1-parameter subgroups, by the classical Hilbert-Mumford criterion, and it turns out that there exists a unique 1-parameter subgroup giving some notion of maximal unstability in the GIT sense. We show how to prove that this special 1-parameter subgroup can be converted into a filtration of the object and coincides with the Harder-Narasimhan filtration, hence both notions of maximal unstability are the same, for the moduli problem of classifying coherent sheaves on a smooth complex projective variety (cf. [GSZ]). A similar treatment can be used to prove the analogous correspondence for holomorphic pairs, Higgs sheaves, rank 2 tensors and finite dimensional quiver representations.

[GSZ] T. Gómez, I. Sols, A. Zamora, A GIT characterization of the Harder-Narasimhan filtration, arXiv:1112.1886v2, (Preprint 2012)

## 10/12/2013, Tuesday, 16:30–17:30

João Pimentel Nunes, Instituto Superior Técnico
Complexified Hamiltonian flows and geodesics on the space of Kahler metrics

We will describe an approach, inspired by geometric quantization, to the complexification of real analytic Hamiltonian flows on Kahler manifolds. The flows correspond to geodesics on the space of Kahler metrics, as suggested by Semmes and Donaldson. We will look at some examples, such as cotangent bundles of compact Lie groups. Based on joint work with J. Mourão.

## 20/12/2013, Friday, 11:30–12:30

Nuno Romão, University of Göttingen
Supersymmetric quantum mechanics on vortex moduli spaces.

I will describe joint work with M. Boekstedt and C. Wegner aiming at uncovering fundamental features of $$N=(2,2)$$ supersymmetric quantum mechanics on moduli spaces of vortices on compact Riemann surfaces, in analogy with the spectrum of quantum dyon-monopole bound states that emerged in connection with Sen's S-duality conjectures in the 1990s. My focus in this talk will be on the (topological A-twisted) supersymmetric Abelian Higgs model coupling to local systems, for both linear and nonlinear targets; the corresponding ground states can be investigated by means of the theory of $$L^2$$-invariants applied to the natural Kaehler metrics on the moduli spaces. I shall explain why the quanta of such Abelian gauge theories can nontrivially realize non-Abelian statistics in particular examples, and motivate a precise conjecture regarding the nonlinear superposition of ground states.