Algebraic Geometry Seminar
Spring 2014 — Tuesdays 3:304:30, location LCB 215
Date  Speaker  Title — click for abstract (if available) 
January 7th 
Luke Oeding Auburn University 
Secant Cumulants and Toric Geometry
We study the secant line variety of the Segre product of projective spaces using special cumulant coordinates adapted for secant varieties. We show that the secant variety is covered by open normal toric varieties. We prove that in cumulant coordinates its ideal is generated by binomial quadrics. We present new results on the local structure of the secant variety. In particular, we show that it has rational singularities and we give a description of the singular locus. We also classify all secant varieties of Segre products that are Gorenstein. Moreover, generalizing (Sturmfels and Zwiernik 2012), we obtain analogous results for the tangential variety.

January 14 
Chunyi Li University of Illinois at Urbana Champaign 
The deformation of Hilb P2 and stability condition
The deformation of the Hilbert scheme of points on the projective plane is studied by Hitchin, Nevins and Stafford via different approaches. I will introduce these constructions, and talk about my recent results on the minimal model program of the deformation of Hilb P2.

January 21  Felix Janda
ETH Zurich 
Tautological relations and cohomological field theories
Tautological classes form a subring of the Chow ring of the moduli space of curves. There is an explicit set of generators but the set of relations remains unknown. In 2012 Aaron Pixton gave an explicit set of conjectural relations and gave a lot of evidence that these are actually all the relations. I want to compare two proofs of the fact that the conjectural relations are actual relations.

January 28 
Emily Clader University of Michigan 
Tautological relations via the orbifold C/Z_r
Tautological classes are certain elements of the cohomology or Chow ring of the moduli space of curves that are important in GromovWitten theory. We describe a method for deriving relations between these classes by studying the GromovWitten theory of the orbifold C/Z_r. Furthermore, we show that the quantum cohomology of C/Z_r is generically semisimple. Using recent ideas of PandharipandePixtonZvonkine, this semisimplicity may be useful for obtaining other tautological relations.

February 4 
Yi Zhu University of Utah 
Iitaka's philosophy and rational curves
Iitaka's philosphy claims that whenever we have a theorem for complete varieties, we should have a countertheorem for open varieties. In this talk, I will give an introduction on this philosophy with examples and evidence. Then I will explain the recent progress on the theory of rational curves on varieties under Iitaka's philosphy. This is a joint work with Qile Chen.

February 11 
Izzet Coskun University of Illinois at Chicago 
BrillNoether divisors in the moduli spaces of sheaves on the plane
I will discuss joint work with Jack Huizenga and Matthew Woolf, where we describe the effective cone of the moduli spaces of semistable sheaves on the plane. The calculation is inspired by Bridgeland stability and hinges on the classification of stable vector bundles on the plane. The fractal nature of the classification and the remarkable numbertheoretic properties of exceptional slopes play an essential role in the calculation.

February 12 (Wednesday) Time/Location: 2pm/LCB 222 
Karl Schwede Penn State University 
Inversion of adjunction for rational and Du Bois pairs
We prove a new inversion of adjunction statement for rational and Du Bois singularities. Roughly speaking, this says that if we have a family over a smooth base with Du Bois special fiber and rational generic fiber, then the total space also has rational singularities. Furthermore, we even generalize this result to the context of rational and Du Bois pairs as defined by Kollár and Kovács. Imprecisely, a pair (X,D) is Du Bois if the failure of X to be Du Bois is equal to the failure of D to be Du Bois. In order to accomplish our inversion of adjunction result we need to prove, for pairs, many recent results on Du Bois singularities. I will describe some of these ideas. This is joint work with Sandor Kovács.

February 18 
Richard Wentworth University of Maryland 
The YangMills flow on Kaehler manifolds
The fundamental work of Donaldson and UhlenbeckYau proves the the smooth convergence of the YangMills flow of stable integrable unitary connections on hermitian vector bundles over Kaehler manifolds. This was generalized by Bando and Siu to incorporate certain (singular) hermitian structures on reflexive sheaves. BandoSiu also conjectured what happens when the initial sheaf is unstable; namely, that the limiting behavior should be controlled by the HarderNarasimhan filtration of the sheaf. In this talk I will describe the solution to this question, which draws on the work of several authors.

February 21 (Friday) Time/Location: 2pm/LCB 215 
Thomas Nevins University of Illinois at UrbanaChampaign 
Hamiltonian reduction in representation theory and algebraic geometry
Hamiltonian reduction arose as a mechanism for reducing complexity of systems in mechanics, but it also provides a tool for constructing complicated but interesting objects from simpler ones. I will illustrate how this works in representation theory and algebraic geometry via examples. I will explain a new structure theory, motivated by Hamiltonian reduction, for some categories (of Dmodules) of interest to representation theorists, and, if there is time, indicate applications to the cohomology of (hyperkahler) manifolds. The talk will not assume that members of the audience know the meaning of any of the abovementioned terms. The talk is based on joint work with K. McGerty.

February 25 
Qile Chen Columbia University 
A^1curves on quasiprojective varieties
The theory of stable log maps was developed recently for studying the degeneration of GromovWitten invariants. In this talk, I will introduce another important aspect of stable log maps as a useful tool for investigating A^1curves on quasiprojective varieties, which are the analogue of rational curves on proper varieties. At least two interesting applications of A^1curves will be introduced in this talk. For classical birational geometry, the A^1curves can be used to produce very free rational curves on Fano complete intersections in projective spaces. On the arithmetic side, A^1connectedness gives a general frame work for the existence of integral points over function field of curves. This is joint work with Yi Zhu.

March 4 
Martin Olsson UC Berkeley 
FourierMukai partners in positive characteristic
I will discuss my joint with work Max Lieblich on FourierMukai partners of K3 surfaces in positive characteristic. In particular, I will discuss the finiteness of the number of FourierMukai partners of a given K3 surface, their realizations as moduli spaces of vector bundles, and various arithmetic applications.

March 11  SPRING BREAK  SPRING BREAK 
March 18 
Brian Osserman UC Davis 
Recent progress on vector bundles with sections
Higherrank BrillNoether, at its most basic, seeks to answer the following
question: how many global sections can a (semistable) vector bundle of
given rank and degree have on general curve of genus g? When there exist
vector bundles with a kdimensional space of sections, one then asks how
many such bundles there are. The classical rank1 version was settled
completely in the 1970's and 1980's, but even the rank2 case is still
wide open, with many partial results but no comprehensive conjecture.
In the 1990's, Bertram, Feinberg and Mukai observed that the canonical
determinant case has special behavior, with a higher expected dimension,
and I will discuss recent work which studies the special determinant case
more systematically. This work is in two parts: producing new expected
dimensions on smooth curves by exploiting symmetries to produce lower
bounds on dimensions, and developing the general theory of higherrank
limit linear series in order to use degeneration arguments to prove
existence results. Some of this is joint work with Montserrat Teixidor i
Bigas.

March 25 
Dragos Oprea UC San Diego 
The Chern classes of the Verlinde bundles
The Verlinde bundles over the moduli space M_g are obtained by considering relative moduli spaces of semistable bundles over smooth curves and associated theta divisors. Their ranks are given by the wellstudied Verlinde numbers. I will discuss a formula for the total Chern character of the Verlinde bundles, as well as extensions over the moduli space of stable curves \overline M_g. This is based on joint work with Alina Marian, Rahul Pandharipande, Aaron Pixton and Dimitri Zvonkine.

April 3 (Thursday) Time/Location: 2pm/LCB 323 
Ionut CiocanFontanine University of Minnesota 
Quasimaps, wallcrossings, and mirror symmetry
Quasimaps provide compactifications, depending on a stability parameter, for moduli spaces of maps from nonsingular algebraic curves to a large class of GIT quotients. These compactifications enjoy good properties and in particular they carry virtual fundamental classes. As the parameter varies, the resulting invariants are related by wallcrossing formulas. I will present some of these formulas in genus zero, and will explain why they can be viewed as generalizations (in several directions) of Givental's toric mirror theorems. I will also describe extensions of wallcrossing to higher genus, and (time permitting) to orbifold GIT targets as well.
The talk is based on joint works with Bumsig Kim, and partly also with Daewoong Cheong and with Davesh Maulik.

April 8 
Joonyeong Won KIAS (Korea) 
Cylinders in del Pezzo surfaces
Let (X,H) be a ample polarization of a projective variety
X. An Hpolar cylinder in X
is an open affine cylinderlike set whose complement is a support of
an effective Qdivisor Qrationally equivalent to H. This notion
relates affine, birational and Kaehler geometry.
we will show how to construct cylinders and to prove nonexistence of
them in smooth and mildly singular del Pezzo surfaces
This is an extended answer of an question of Zaidenberg and Flenner in 2003.
This is a joint work with I.Cheltsov and J.Park.

April 15 
Mathieu Huruguen University of British Columbia 
Special reductive groups over an arbitrary field
A linear algebraic group G defined over a field k is called special if every Gtorsor over every field extension of k is trivial. In a modern language, it can be shown that the special groups are those of essential dimension zero. In 1958 Grothendieck classified special groups in the case where the base field k is algebraically closed. In this talk I will explain some recent progress towards the classification of special reductive groups over an arbitrary field. In particular, I will give the classification of special semisimple groups, special reductive groups of inner type and special quasisplit reductive groups over an arbitrary field k.

April 22 
Enrico Arbarello Università di Roma La Sapienza 
Moduli of sheaves on a K3 surface and quiver varieties
In a joint work with Giulia Saccà, we study the singularities of
moduli spaces of rank 0 sheaves on K3 surface; we prove,
in a number of cases, the formality of their Kuranishi family; and we
relate their symplectic resolutions to the ones of Nakajima's quiver varieties
coming from variations of GIT quotients.

Archive of previous seminars.
This web page is maintained by Sofia Tirabassi and Yi Zhu.