1999 seminars

Florian Schaetz, Instituto Superior Técnico
Integrating representations up to homotopy

I report on joint work with C. Arias Abad on the integration of representations up to homotopy (including reps of Lie algebras and Quillen's flat superconnections). The heart of the construction is an homotopy equivalence between the dg algebra of differential forms and the dg algebra of simplicial cochains due to Gugenheim.

Ana Cristina Ferreira, Universidade do Minho
Three-forms and moduli spaces of instantons

In this talk we will discuss some aspects of Riemannian geometry where the chosen connection has a nonzero three-form as its torsion tensor. We will show how to decompose the curvature tensor for such a connection in four dimensions and mention how this motivates our definition of Einstein manifolds with skew torsion. Relations with Einstein-Weyl and Hermitian geometry will also be presented. We will also consider the link with instanton moduli spaces and see how to a three form on the base manifold induces a three-form on the moduli space and give particular attention to the case where the base manifold is the four-sphere with the round metric.

Jihun Park, Pohang University of Science and Technology
Kahler-Einstein metrics on Fano manifolds

Tian and Yau have introduced alpha-invariants in order to study Kahler-Einstein metrics on Fano manifolds. Later, Kollar and Demailly showed that the alpha-invariant can be translated into log canonical threshold, an important invariant in birational geometry. I will briefly explain the relation between Kahler-Einstein metric and log canonical threshold and then study examples to show how to compute the alpha-invariants using log canonical thresholds.

Jaume Amoros, UPC-Barcelona
Holomorphic tangent vector fields in compact Kahler and projective manifolds

The birational classification of projective manifolds with holomorphic tangent vector fields was developed by F. Severi, R. Hall and D. Lieberman. The Albanese mapping allows one to make this classification biholomorphic in the non-uniruled case, extending Calabi‘s structure theorem for varieties with trivial canonical bundle. These results may be extended to compact Kähler manifolds, using small deformations of the complex structure. They show that the study of the dynamics of holomorphic vector fields in them reduces to the case of rational varieties.

Andres Pedroza, Colima University, Mexico
On the Bounded Isometry Conjecture

F. Lalonde and L. Polterovich study the isometries of the group of Hamiltonian diffeomorphisms with respect to the Hofer metric. They defined a symplectic diffeomorphism $\phi$ to be bounded, if the Hofer norm of $[\phi, h]$ remains bounded as $h$ varies on $\operatorname{Ham}(M, \omega)$. The set of bounded symplectic diffeomorphisms, $BI_0 (M, \omega),$ of $(M, \omega)$ is a group that contains all Hamiltonian diffeomorphisms. They conjectured that these two groups are equal, $\operatorname{Ham}(M, \omega) = BI_0 (M, \omega)$ for every closed symplectic manifold. They prove this conjecture in the case when the symplectic manifold is a product of closed surfaces of positive genus. In this talk we give an outline of a new class of manifolds for which bounded isometry conjecture holds.

Miguel Abreu, Instituto Superior Técnico
Remarks on Lagrangian Intersections in Toric Manifolds

I will show how to get a lot of "mileage" out of applying two simple geometric remarks to questions on Lagrangian intersections in toric manifolds. Joint work with Leonardo Macarini.

Fiammetta Battaglia, University of Florence
The Betti numbers of the Penrose kite

We will define the Betti numbers of the generalized toric spaces associated to nonrational simple convex polytopes and show that they depend on the combinatorial type of the polytope exactly as they do in the rational case. We will illustrate this result by focussing on a particular example of simple non rational convex polytope: the Penrose kite.

References

  • F. Battaglia, E. Prato, The Symplectic Penrose Kite, Commun. Math. Phys., 299, (2010), Number 3, 577-601.
  • F. Battaglia, Betti numbers of the geometric spaces associated to nonrational simple convex polytopes, Proc. Amer. Math. Soc. 139 (2011), 2309-2315.

Luca Migliorini, University of Bologna
Topological properties of a class of algebraically completely integrable systems with $C^*$ action

I will discuss two examples, one arising from non-abelian Hodge theory, the other from Hilbert schemes of surfaces, of algebraically completely integrable systems with $C^*$ action, showing an as yet not understood behaviour. In both cases there exists another holomorphic symplectic variety whose Hodge theory reflects some topological properties of the integrable systems, more precisely, the weight filtration on the cohomology of this latter variety coincides up to a trivial renumbering with a topological filtration associated with the “integrable system map” (a variant of the Leray filtration, called the perverse Leray filtration). Joint work with M. de Cataldo, T. Hausel.

Philippe Monnier, Toulouse
Rigidity of Hamiltonian actions

In this talk, I will state an abstract normal form theorem and explain how to apply it in order to prove the rigidity of Hamiltonian actions.

Camilo Arias-Abad, Zurich
Reidemeister torsion for flat superconnections

I will begin by describing the Cheeger-Müller theorem. This is the fact that the Reidemeister torsion — a combinatorial invariant of odd dimensional manifolds — is equal to the analytic torsion defined by Ray and Singer. I will then explain how higher dimensional parallel transport, constructed via the $A_\infty$ version of de Rham\'s theorem, can be used to define Reidemeister torsion for flat superconnections.

This talk is based on joint work in progress with F. Schätz.

José Mourão, Instituto Superior Técnico
Examples of complexification of Lagrangian fibrations and quantization

I will recall some of the difficulties in realizing quantization as a functor. In the framework of geometric quantization, one of such difficulties is associated with the dependence of quantization on the choice of a complex structure on the symplectic manifold. I will describe some natural infinite dimensional families of Kaehler structures degenerating to Lagrangian fibrations on cotangent bundles of compact Lie groups and to the singular $T^n$ fibration on toric manifolds. Coming from the other side these families of Kaehler structures can be viewed as complex time evolutions of the Lagrangian fibrations. This point of view, together with an appropriate uniqueness theorem, allows us to shed new light on the coherent state transform for compact Lie groups and on its relation with geometric quantization. This talk is based on joint work in progress with Will Kirwin and João P. Nunes.

Cornelia Vizman, University of Timisoara
Current algebra functors

Two current algebra functors were introduced by Alekseev and Severa: they assign Lie algebras to a pair consisting of a smooth manifold $M$ and a differential graded Lie algebra $A$. Important extensions of current algebras can be obtained via those current algebra functors: all central extensions by the fundamental current algebra cocycles (including affine Lie algebras on the circle), the Fadeev-Mickelsson-Sahtashvili abelian extension, and the Lie algebra of symmetries for the sigma model. Finally we present groups integrating Lie algebras obtained via current algebra functors.

Luís Diogo, Stanford
Floer homology and holomorphic curves

Floer homology is a powerful tool in symplectic geometry. It is often hard to compute it explicitly, because it involves counting solutions of a perturbed Cauchy-Riemann equation. I will explain an approach to showing how Floer homology can instead be described (under certain hypotheses) by counting solutions of (unperturbed) Cauchy-Riemann equations, as well as gradient flow lines of auxiliary Morse functions. The advantage is that this information can sometimes be computed explicitly. This framework seems adequate for studying spectral invariants, symplectic homology and Floer-type operations. This is part of a joint project with S. Borman, Y. Eliashberg, S. Lisi and L. Polterovich.

Matthew Borman, U. Chicago
Symplectic reduction of quasi-morphisms and quasi-states

Symplectic quasi-states are a convenient way to package and see various rigidity phenomenon in symplectic topology. Their general construction uses Hamiltonian Floer homology and requires that the quantum homology ring contains a field summand. Since quantum homology is not functorial there is no general algebraic way to create new quasi-states from known examples. In this talk I will explain how symplectic reduction provides a sort of geometric functoriality that allows quasi-states to descend along to symplectic reductions without further quantum homology computations.

William Kirwin, Instituto Superior Técnico
Momentum space for compact Lie groups

Let $K$ be a compact Lie group. As is well known, $L^2(K)$ can be interpreted as the “position-space” geometric quantization of the cotangent bundle $T^*K$. In this talk, I will describe a “momentum-space” quantization of $T^*K$. I will also explain how this momentum-space quantization is linked to the position-space representation via parallel transport with respect to the Axelrod-della Pietra-Witten-Hitchin connection in a certain Hilbert bundle. In particular, it is a result of Florentino-Matias-Mourao-Nunes that parallel transport along a particular geodesic from position space to an intermediate fiber is the generalized Segal-Bargmann transform. I will explain how their result can be extended to any other interior fiber (thus obtaining generalized Segal-Bargmann transforms for more general complex structures), and moreover that when extended to momentum space, parallel transport from position space yields the Peter-Weyl decomposition. This is joint work with S. Wu.

Florent Schaffhauser, Universidad de Los Andes, Colombia
Topology of moduli spaces of vector bundles on a real algebraic curve

Moduli spaces of real and quaternionic vector bundles on a curve can be expressed as Lagrangian quotients and embedded into the symplectic quotient corresponding to the moduli variety of holomorphic vector bundles of fixed rank and degree on a smooth complex projective curve. From the algebraic point of view, these Lagrangian quotients are irreducible sets of real points inside a complex moduli variety endowed with an anti-holomorphic involution. This presentation as a quotient enables us to generalise the equivariant methods of Atiyah and Bott to a setting with involutions, and compute the mod 2 Poincaré series of these real algebraic varieties. This is joint work with Chiu-Chu Melissa Liu.

Stephane Guillermou, Grenoble
Microlocal theory of sheaves and link with symplectic geometry

In the talk I will explain a joint work with M. Kashiwara and P. Schapira, inspired by results of D. Tamarkin. The microsupport of a sheaf on a manifold $M$, introduced by Kashiwara and Schapira, is a closed conic subset of the cotangent bundle of $M$ which indicates how far the sheaf is from being locally constant. It can be used to translate symplectic diffeomorphisms of the cotangent bundle into operations on sheaves on the base. In the talk I will recall quickly the definition and properties of the microsupport and explain how it can be used to recover non-displaceability results (Arnold’s conjecture).

Bruno Oliveira, U. Miami
Closed symmetric differentials of rank $1$ and $2$ on surfaces and their geometry

The presence of holomorphic 1-forms on a compact kahler manifold $X$ implies topological properties of $X$. Moreover, from their presence also follows the existence of a holomorphic map from $X$ into a complex torus from which all the holomorphic $1$-forms of $X$ are induced from. The talk gives a complete extension of this result to symmetric differentials of rank $1$. This result belongs to the program whose aim is to understand the class of symmetric differentials that have a close to topological nature (symmetric differentials of rank $1$ will be shown to be closed symmetric differentials). We will also discuss these results for degree $2$.

Paul Norbury, U. Melbourne
Gromov-Witten invariants of $\mathbb{P}^1$ and Eynard-Orantin invariants

Eynard and Orantin have recently defined invariants of any compact Riemann surface equipped with two meromorphic functions, as a tool for studying enumerative problems in geometry. I will descibe how these invariants bring new insight into the well-studied problem of the Gromov-Witten invariants of $\mathbb{P}^1$.

Margarida Mendes Lopes, Instituto Superior Técnico
A characterization of the symmetric square of a curve

Very often the existence of curves with special features on complex surfaces imposes constraints on the properties of the surface. In this seminar, after explaining some of the properties of projective complex surfaces with irregularity $q \gt 0$ (i.e. possessing $q$ independent holomorphic $1$-forms), I will explain how the existence of certain curves characterizes the symmetric square of a curve.

Agnès Gadbled, Cambridge
Exotic monotone Lagrangian tori

There exist two constructions of families of exotic monotone Lagrangian tori in complex projective spaces and products of spheres, namely the one by Chekanov and Schlenk and the one via the Lagrangian circle bundle construction of Biran. It was conjectured that these constructions give Hamiltonian isotopic tori. I will explain why this conjecture is true in the complex projective plane and the product of two two-dimensional spheres.

Silvia Sabatini, École Polytechnique Fédérale de Lausanne
A necessary condition for $S^1$-Hamiltonian actions with isolated fixed points

For every compact symplectic manifold $M$ with a Hamiltonian circle action and isolated fixed points, a simple algebraic identity involving the first Chern class is derived. When the manifold satisfies an extra “positivity condition”, this enables us to construct an algorithm to obtain linear relations among the isotropy weights at the fixed points. If $\dim(M)$ is less than or equal to 6 and the number of fixed points is minimal, this algorithm quickly determines all the possible families of isotropy weights, simplifying the proofs due to Ahara and Tolman. In this case all the possible equivariant cohomology rings and Chern classes are determined.

Daniele Sepe, University of Leicester, UK
An affine obstruction to constructing Lagrangian bundles

Lagrangian fibrations arise naturally in the study of Liouville integrable systems and can be used to construct topological and symplectic invariants of such dynamical systems. As shown by Weinstein and Duistermaat amongst others, Lagrangian fibrations (and, more generally, foliations) are connected with affinely flat geometry, i.e. the differential geometry of those manifolds which admit a flat, torsion-free connection. In this talk, the problem of constructing Lagrangian bundles over a fixed manifold is discussed using affinely flat geometry; it is proved that the obstruction to constructing examples with non-trivial topological invariants is determined the radiance obstruction, an important cohomological invariant of affinely flat manifolds introduced by Goldman and Hirsch. Time permitting, I will illustrate how to extend this theory to the case with singularities.

Johan Martens, Univ. Aahrus
Compactifications of reductive groups as moduli stacks of bundles

We will introduce a class of moduli problems for any reductive group G, whose moduli stacks provide us with (toroidal) equivariant compactifications of G. Morally speaking, the objects in the moduli problem could be thought of as stable maps of a twice-punctured sphere into the classifying stack BG. More precisely, they consist of $G_m$ equivariant $G$-principal bundles on chains of projective lines, framed at the extremal poles. The choice of a fan determines a stability condition. All toric orbifolds are special cases of these, as are the "wonderful compactifications" of semi-simple groups of adjoint type constructed by De Concini-Procesi. Our construction further provides a canonical orbifold compactification for any semi-simple group. From a symplectic point of view, these compactifications can be understood as non-abelian cuts of the cotangent bundle of a maximal compact subgroup. This is joint work with Michael Thaddeus (Columbia).