Humberto Diaz : On Chow groups of Varieties
- Algebraic Geometry ( 114 Views )For a complex algebraic variety, the Chow group is a geometric invariant which is easy to construct but often difficult to compute. In this talk, I will describe the construction of the Chow group, give some key examples and discuss some difficult open questions. I will also present a result about the Chow group of 0-cycles of the surface which parametrizes lines on a cubic 3-fold.
Chad Schoen : Chow groups, an introduction
- Algebraic Geometry ( 124 Views )Chow groups give functors from algebraic varieties to abelian groups which are related to (co)homology. However Chow groups frequently contain more information than (co)homology. The construction of Chow groups is easy. Their computation is often difficult. This talk has two aims. First of all it will serve as an introduction to Chow groups which should be accessible to those who have taken a one semester course in Riemann surfaces, two semesters of algebraic topology, and have a passing acquaintance with affine and projective algebraic varieties. (One month in an algebraic geometry course may suffice for the latter.) Given that the next two talks in the algebraic geometry seminar will discuss various aspects of Chow groups, this talk may function as a warm up. The second aim is to introduce Bloch's conjecture on the Chow group of zero dimensional algebraic cycles on a non-singular projective surface. Throughout the talk one may assume that the base field is the complex numbers.
Ben Howard : Twisted Gross-Zagier theorems and central derivatives in Hida families
- Algebraic Geometry ( 154 Views )Abstract: Given a Hida family of modular forms, a conjecture of Greenberg predicts that L-functions of forms in the family should generically vanish to order 0 or 1 at the center of the functional equation. Similarly the Selmer groups of forms in the family should generically be of rank 0 or 1. In this talk I will prove a generalization of the Gross-Zagier theorem, relating Neron-Tate heights of special points on the modular Jacobian J_1(N) to derivatives of L-functions, and explain how this generalization can be used to verify Greenberg's conjecture for any particular Hida family.
Tatsunari Watanabe : Weighted completion and Generic curves in positive characteristics
- Algebraic Geometry ( 134 Views )It follows from results in Teichmüller Theory that generic curves of type (g,n) in characteristic zero have only n rational points that come from the tautological points. Richard Hain gave an algebraic proof of the theorem. Extending his algebraic method to positive characteristics, we prove the analogous result for generic curves in positive characteristics. The primary tool used is the theory of weighted completion, which was developed by Richard Hain and Makoto Matsumoto. It linearises a profinite group such as arithmetic mapping class groups. In our case, the weighted completion connects topology and algebraic geometry in positive characteristics.
Bhargav Bhatt : Interpolating p-adic cohomology theories
- Algebraic Geometry ( 298 Views )Integration of differential forms against cycles on a complex manifold helps relate de Rham cohomology to singular cohomology, which forms the beginning of Hodge theory. The analogous story for p-adic manifolds, which is the subject of p-adic Hodge theory, is richer due to a wider variety of available cohomology theories (de Rham, etale, crystalline, and more) and torsion phenomena. In this talk, I will give a bird's eye view of this picture, guided by the recently discovered notion of prismatic cohomology that provides some cohesion to the story. (Based on joint work with Morrow and Scholze as well as work in progress with Scholze.)
Jeffrey Giansiracusa : Equations of tropical varieties
- Algebraic Geometry ( 128 Views )Tropical geometry is a combinatorial shadow of algebraic geometry over a nonarchimedean field that encodes information about things like intersections and enumerative invariants. Usually one defines tropical varieties as certain polyhedral subsets of R^n satisfying a balancing condition. I'll show how these arise as the solution sets to certain systems of polynomial equations over the tropical semiring T = (R union -infinity, max, +) related to matroids. This yields a notion of tropical Hilbert polynomials, and in this framework there is a universal tropicalization that is closely related to the Berkovich analytification and the moduli space of valuations.
Olivier Debarre : Fake projective spaces and fake tori
- Algebraic Geometry ( 150 Views )We discuss compact complex manifolds which ``look like'' complex projective spaces or complex tori. Hirzebruch and Kodaira proved in 1957 that when n is odd, any compact Kähler manifold X which is homeomorphic to Pn is isomorphic to Pn. This holds for all n by Aubin and Yau's proofs of the Calabi conjecture. One may conjecture that it should be sufficient to assume that the integral cohomology rings H*(X,Z) and H*(Pn,Z) are isomorphic.
Catanese observed that complex tori are characterized among compact Kähler manifolds X by the fact that their integral cohomology rings are exterior algebras on H1(X,Z) and asked whether this remains true under the weaker assumption that the rational cohomology ring is an exterior algebra on H1(X,Q). (We call the corresponding compact Kähler manifolds ``rational cohomology tori".) We give a negative answer to Catanese's question by producing explicit examples. We also prove some structure theorems for rational cohomology tori. This is work in collaboration with Z. Jiang, M. Lahoz, and W. F. Sawin.
John Calabrese : Gabriels theorem and points
- Algebraic Geometry ( 120 Views )An old theorem of Gabriel says that a variety X can be reconstructed by the category Coh(X) of coherent sheaves on it. This result has seen a few generalizations over the years. I will present a different and more geometric proof, with new generalizations. The idea being that X can be recovered as a moduli space of "points" in Coh(X). This is joint work with Michael Groechenig.
Anders Buch : Quantum cohomology of isotropic Grassmannians
- Algebraic Geometry ( 161 Views )The (small) quantum cohomology ring of a homogeneous space is a deformation of the classical cohomology ring, which uses the three point, genus zero Gromov-Witten invariants as its structure constants. I will present structure theorems for the quantum cohomology of isotropic Grassmannians, including a quantum Pieri rule for multiplication with the special Schubert classes, and a presentation of the quantum ring over the integers with the special Schubert classes as the generators. These results are new even for the ordinary cohomology of isotropic Grassmannians, and are proved directly from the definition of Gromov-Witten invariants by applying classical Schubert calculus to the kernel and span of a curve. This is joint work with A. Kresch and H. Tamvakis.
Julie Rana : Moduli of general type surfaces
- Algebraic Geometry ( 183 Views )It has been 30 years since Koll\ar and Shepherd-Barron published their groundbreaking paper describing a compactification of Giesekers moduli space of surfaces of general type. As with all compactifications, the work raised natural questions. What is the structure of these moduli spaces and the boundary in particular? What sorts of singularities might we expect to obtain? What types of surfaces give rise to divisors in the moduli space, and are these divisors smooth? We discuss general results bounding types of Wahl singularities, and use them to address these questions in the context of Horikawa-type surfaces.
Sebastian Casalaina-Martin : Distinguished models of intermediate Jacobians
- Algebraic Geometry ( 178 Views )In this talk I will discuss joint work with J. Achter and C. Vial showing that the image of the Abel--Jacobi map on algebraically trivial cycles descends to the field of definition for smooth projective varieties defined over subfields of the complex numbers. The main focus will be on applications to topics such as: descending cohomology geometrically, a conjecture of Orlov regarding the derived category and Hodge theory, and motivated admissible normal functions.
Giulia Sacca : Compact Hyperkahler manifolds in algebraic geometry
- Algebraic Geometry ( 116 Views )Hyperkahler (HK) manifolds appear in many fields of mathematics, such as differential geometry, mathematical physics, representation theory, and algebraic geometry. Compact HK manifolds are one of the building blocks for algebraic varieties with trivial first Chern class and their role in algebraic geometry has grown immensely over the last 20 year. In this talk I will give an overview of the theory of compact HK manifolds and then focus on some of my work, including a recent joint work with R. Laza and C. Voisin.
Angelo Vistoli : Fundamental gerbes
- Algebraic Geometry ( 122 Views )Let X be a connected and geometrically reduced variety over a field k, with a fixed rational point x_0 in X(k). Nori defined a profinite group scheme N(X,x_0), usually called Nori's fundamental group scheme, with the property that homomorphisms N(X,x_0) to a fixed finite group scheme G correspond to G-torsors P--> X with a fixed rational point in the inverse image of x_0 in P. If k is algebraically closed of characteristic 0 this coincides with Grothendieck's fundamental group, but is in general very different. Nori's main theorem is that if X is complete, the category of finite-dimensional representations of N(X,x_0) is equivalent to an abelian subcategory of the category of vector bundles on X, the category of essentially finite bundles. In my talk I will recall the basics of the theory of group schemes and torsors, and give a detailed description of Nori's results. Then I will explain my work in collaboration with Niels Borne, from the University of Lille, in which we extend them by removing the dependence on the base point, substituting Nori's fundamental group with a gerbe (in characteristic 0 this had already been done by Deligne), and give a simpler definition of essentially finite bundle, and a more direct and general proof of the correspondence between representations and essentially finite bundles.
Ravindra Girivaru : Lefschetz type theorems for algebraic cycles and vector bundles.
- Algebraic Geometry ( 190 Views )The Weak Lefschetz theorem (or the Lefschetz hyperplane theorem) states that for a smooth, projective variety Y and a smooth hyperplane section X in Y, the restriction map of cohomologies H^i(Y) to H^i(X) is an isomorphism for i less than dim{X}, and an injection when i equal to dim{X}. Analogues of this theorem have been conjectured for algebraic cycles. We will talk about some results in this area. We will also talk about such questions for vector bundles.
Jimmy Dillies : On some K3 automorphisms
- Algebraic Geometry ( 171 Views )In order to construct a viable model of string theory, one seeks to build Calabi Yau threefolds with prescribed conditions. Borcea and Voisin were able to built a family of Calabi-Yau threefolds using elliptic curves and K3 surfaces admitting non symplectic involutions. We will display how the construction can be generalized by studying higher order non symplectic automorphisms on K3 surfaces
Patricia Hersh : Topology and combinatorics of regular CW complexes
- Algebraic Geometry ( 144 Views )Anders Björner characterized which finite, graded partially ordered sets (posets) are closure posets of finite, regular CW complexes, and he also observed that a finite, regular CW complex is homeomorphic to the order complex of its closure poset. One might therefore hope to use combinatorics to determine topological structure of stratified spaces by studying their closure posets; however, it is possible for two different CW complexes with very different topological structure to have the same closure poset if one of them is not regular. I will talk about a new criterion for determining whether a finite CW complex is regular (with respect to a choice of characteristic functions); this will involve a mixture of combinatorics and topology. Along the way, I will review the notions from topology and combinatorics we will need. Finally I will discuss an application: the proof of a conjecture of Fomin and Shapiro, a special case of which says that the Schubert cell decomposition of the totally nonnegative part of the space of upper triangular matrices with 1's on the diagonal is a regular CW complex homeomorphic to a ball.
Will Donovan : Noncommutative deformations and the birational geometry of 3-folds
- Algebraic Geometry ( 129 Views )I will speak about recent work with Michael Wemyss (arXiv:1309.0698), applying noncommutative deformation theory to study the birational geometry of 3-folds. I will give a brief introduction to noncommutative deformations, and explain how every flippable or floppable rational curve in a 3-fold has a naturally associated algebra of noncommutative deformations. This construction yields new information about the (commutative) geometry of the 3-fold, and provides a new tool to differentiate between flops. As a further application, we show how the noncommutative deformation algebra controls the homological properties of a floppable curve, relating a Fourier-Mukai flop-flop functor and a spherical twist about the universal family over the noncommutative deformation algebra. I will also explain work in progress applying this approach to other geometric situations, and to higher dimensions.
Melanie Matchett Wood : Motivic Discriminants
- Algebraic Geometry ( 147 Views )We consider the "limiting behavior" of *discriminants* (or their complements), by which we mean informally the closed locus in some parameter space of some type of object where the objects have singularities. We focus on the collection of unordered points on a variety X, and linear systems on X. These are connected --- we use the first to understand the second. We describe their classes in the Grothendieck ring of varieties, as the number of points gets large, or as the line bundle gets very positive. As applications, (i) we show the motivic analogue of Poonen's point-counting result: the motivic probability of a section of L being smooth (as L gets large) is 1 / Z_X( \A^{-\dim X - 1} ) (where Z_X is the motivic zeta function), and (ii) show a priori unexpected structure in configuration spaces of points on a variety, with topological and point-counting consequences. Some low-tech examples: if v is a partition of n \leq 9, and v \neq (1,1,2,2,3), then the v-discriminant in the space of degree n polynomials (those polynomials with those root multiplicities, or worse) can be cut-and-pasted into affine space. (Question: over \C, does the complement have only two nonvanishing cohomology groups? What structure remains when n is larger?) This is joint work with Ravi Vakil.
Thomas Haines : A Tannakian approach to Bruhat-Tits buildings and parahoric group schemes
- Algebraic Geometry ( 165 Views )For the general linear group, the Bruhat-Tits building can be realized explicitly in terms of periodic lattice chains in the standard representation. Further, each parahoric group scheme can be described as an automorphism group of a particular chain. I will explain a Tannakian formalism which establishes analogous descriptions for arbitrary connected reductive groups over complete discretely valued fields. This complements previously known results for classical groups, and fits in with Mumford's Geometric Invariant Theory, where spherical buildings are similarly described. This is joint work with Kevin Wilson.
Yiannis Sakellaridis : Moment map and orbital integrals
- Algebraic Geometry ( 259 Views )In the Langlands program, it is essential to understand spaces of Schwartz measures on quotient stacks like the (twisted) adjoint quotient of a reductive group. The generalization of this problem to spherical varieties calls for an understanding of the double quotient H\G/H, where H is a spherical subgroup of G. This has been studied by Richardson for symmetric spaces. In this talk, I will present a new approach, for spherical varieties "of rank one", based on Friedrich Knop's theory of the moment map and the invariant collective motion.