Bianca Viray (Berkeley)

Failure of the Hasse principle for Enriques surfaces

Most counterexamples to the Hasse principle can be explained by an algebraic Brauer-Manin obstruction. We define some of the other possible obstructions and exhibit an Enriques surface where the failure of the Hasse principle is not explained by an algebraic Brauer-Manin obstruction. We explain the difficulty that arises in studying the transcendental Brauer-Manin obstruction and describe work in progress in this vein.

Nov. 15, 2009

David Smyth (Harvard)

Modular compactifications of *M _{g,n}*

A modular compactification of

Nov. 14, 2009

Hsian-Hua Tseng (Ohio State)

Gromov-Witten theory for root stacks

Let *r* be a positive integer. Given a line bundle *L* over a space *X*
there is a stack over *X* which classifies *r*-th
roots of *L*. This stack of *r*-th roots of *L*, which is a gerbe over
*X* banded by the cyclic group of order *r*, appears naturally in various places
such as the structure result of toric Deligne-Mumford stacks. The goal of this talk is to
discuss recent progresses towards understanding Gromov-Witten theory of such stacks of
roots (joint work with E. Andreini and Y. Jiang).

Nov. 14, 2009

Michael Zieve (Michigan)

Automorphism groups of curves

Hurwitz proved that a complex algebraic curve of genus *g*>1 has at most
84(*g*-1) automorphisms. In case equality holds, the automorphism group has a
quite special structure. However, in a qualitative sense, all finite groups *G*
behave the same way: the least *g*>1 for which *G* acts on a genus-*g*
curve is on the order of (#*G*)·*d*(*G*), where
*d*(*G*) is the minimal number of generators of *G*. I will present joint
work with Bob Guralnick on the analogous question in positive characteristic. In this
situation, certain special families of groups behave fundamentally differently from all
others. If we restrict to *G*-actions on curves with ordinary Jacobians, we obtain a
precise description of the exceptional groups and curves.

Nov. 14, 2009

Sabin Cautis (Columbia)

Equivalences via geometric sl(2) actions

I will explain how sl(2) actions can be used to construct equivalences between
derived categories of coherent sheaves. The example we consider are the derived
categories of coherent sheaves on cotangent bundles to Grassmannians. Our construction
generalizes Seidel-Thomas twists.

Oct. 7, 2009

Daniel Erman (Berkeley)

Deformations of zero-dimensional schemes

A natural question in the study of zero-dimensional schemes is
to determine when such a scheme deforms to a disjoint union of points.
I will discuss a syzygetic invariant which yields sharp information
about this question. I will also give applications to the study of
Hilbert schemes of points. This is joint work with M. Velasco.

Sep. 23, 2009

Tommaso De Fernex (Utah)

Rigidity properties of Fano varieties

I will discuss some deformation properties of Fano varieties. The general methods rely on the
investigation of the variation of the cone of effective curves and, more generally, of the Mori
chamber decomposition, which, according to Mori theory, encode information on the geometry of the
variety. The talk is based on joint work with C. Hacon.

Sep. 16, 2009

Mark DeCataldo (Stony Brook)

Topology of the Hitchin fibration and Hodge theory of character varieties

Given a compact Riemann surface *X* of genus at least two, there are two
algebraic varieties attached to it: the character variety *Ch*, and the
Hitchin moduli space *M*.
The non Abelian Hodge theorem asserts that they are diffeomorphic (but have
different complex structures). While the rational cohomology rings *H**Ch*)
and *H**M*) are isomorphic, the mixed Hodge structures are different and
so are the weight filtrations, which therefore cannot possibly correspond via
the non Abelian Hodge theorem.
In recent joint work with T. Hausel (Oxford) and L. Migliorini (Bologna) it is shown that the
non Abelian Hodge theorem exchanges the weight filtration on *H**Ch*)
with the (perverse) Leray filtration on *H**M*) for the Hitchin map
*h*: *M*→**C**^{n}. Moreover, curious symmetries observed on
*H**Ch*) by number-theoretic means, turn out to be the more familiar
Lefschetz and Poincaré symmetries for the map *h*.
(The perverse Leray filtration is formally analogous to the Leray filtration associated with the Leray
spectral sequence. However, as my on-line thesaurus states: "perverse = resistant to guidance or
discipline.")

Apr. 5, 2000

Mihnea Popa (UMich)

Generalized theta linear series on moduli spaces of vector bundles on curves

The main theme will be some results on effective bounds for global generation and normal
generation of multiples of generalized theta line bundles on moduli spaces of vector
bundles on a smooth projective curve *X*. The techniques involved are of two
different flavors: on one hand I will use a result (of independent interest) giving an
optimal upper bound on the dimension of the Hilbert scheme of coherent quotients of a
fixed vector bundle, while on the other hand I will appeal to general vector bundle
techniques on abelian varieties via the notion of Verlinde bundle on the Jacobian of
*X*.

Apr. 3, 2000

David Ben-Zvi (Chicago)

Moduli of curves and bundles via vertex algebras

We will examine some relations between the moduli space of bundles on a fixed algebraic
curve and the moduli space of curves. It turns out that vertex algebras provide a powerful
tool to examine the local geometry of these spaces. A basic element of the theory is the
Sugawara construction, which relates the Virasoro and Kac-Moody algebras ("rotating the
loop in a loop group"). We present a general geometric formulation of the Sugawara
construction, which specializes to the well-known relations between curves and bundles
(Hitchin system, isomonodromy equations, Beilinson-Drinfeld operators and heat equations
for theta functions) as well as to new connections.

Mar. 15, 2000

Brendan Hassett (Chicago)

Ample divisors on holomorphic symplectic fourfolds

The Picard group of a polarized K3 surface *S* may be regarded as an integral
quadratic form with respect to the intersection pairing. There is a dictionary between
the geometry of *S* and the arithmetic properties of this form. For example, there
are criteria for the existence of smooth rational curves in terms of the integers
represented by the form. This yields a simple arithmetic description of the ample cone
of *S*.
Our goal is to extend this dictionary to certain higher dimensional analogs to K3 surfaces,
known as holomorphic symplectic manifolds. These include punctual Hilbert schemes (i.e.,
desingularized symmetric products) of K3 surfaces. We give a conjectural framework
generalizing the picture for K3 surfaces and prove the conjectures in certain cases.
The variety parametrizing lines contained in a cubic fourfold *X* is a holomorphic
symplectic manifold. Our conjectures predict the existence and nonexistence of certain
ruled surfaces on *X* and these surfaces induce unirational parametrizations for
*X*. Hence the rich projective geometry of cubic fourfolds provides a useful
laboratory where we may test our ideas.

Jan. 26, 2000

Tom Nevins (Chicago)

Moduli spaces of framed sheaves on ruled surfaces

I will discuss some moduli spaces of framed torsion-free
sheaves on ruled surfaces; these spaces arise in the study
of algebras of Hecke operators coming from the geometric
Langlands program for surfaces. I use a natural *S*^{1} action
on these moduli spaces to investigate their topology, and
in particular I obtain a complete characterization of their
rational homology for a class of ruled surfaces over elliptic
curves. These computations use ideas from the study of betti
numbers of Hilbert schemes and the study of the topology of
configuration spaces.

Nov. 22, 1999

Sasha Orlik (Cologne)

The cohomology of period domains over finite fields

Period domains are open subsets of generalized flag varieties,
which are described by semistable conditions. In the case of a
local ground field these period domains are open admissible
subsets in the sense of rigid algebraic geometry, whereas in
the finite ground field case one gets open subvarieties. The
goal of the talk is to present the strategy of the compution
of the l-adic cohomology of period domains. The result
confirms a conjecture of Kottwitz and Rapoport.

Nov. 17, 1999

Dennis Gaitsgory

The appearance of the Langlands dual group

In this lecture we will be concerned with the (so far unique) geometric construction of
the Langlands dual group. This construction lies in the core of the whole field.

Nov. 10, 1999

Martin Sombra (IAS)

Effective Nullstellensatz, division formulas, and all that

I will present sharp estimates for the degree and the height of the polynomials in the
Nullstellensatz over the integers. This result improves previous work on this area of
Berenstein-Yger and Krick-Pardo. I will also present an arithmetic Nullstellensatz for
sparse polynomial systems.

(Joint work with T. Krick (Univ. Buenos Aires, Argentina) and L. M. Pardo (Univ. Cantabria, Spain).)

Nov. 9, 1999

Michael Duff (UM Physics)

A layman's guide to M-theory

Superunification of the fundamental interactions underwent a major paradigm shift in 1984
when eleven-dimensional supergravity was knocked off its pedestal by ten-dimensional
superstrings. 1995 witnessed another shift of equal proportions, however, when
superstrings were themselves superseded by "M-theory", a non-perturbative theory which
describes extended objects with two dimensions (supermembranes) and five dimensions
(superfivebranes), which subsumes all five consistent string theories and whose low-energy
limit is, ironically, eleven-dimensional supergravity.

Nov. 2, 1999

Eleny Ionel (Wisconsin)

Gromov-Witten invariants of symplectic sums and applications

The natural cut-and-paste operation for symplectic manifolds is the symplectic sum along a
codimension 2 symplectic submanifold. In this talk we will describe a gluing formula for
Gromov-Witten invariants of the symplectic sum in terms of the relative GW invariants of
the two pieces. This degeneration formula (which is joint work with Tom Parker) describe
s what happens to holomorphic curves as one pinches the neck and explains how to compute
the GW invariant from the limiting curves.
The degeneration formula has plenty of applications, ranging from the construction of
infinitely many exotic symplectic structures on the same smooth manifold to solving
enumerative questions in algebraic geometry. In the second part of the talk I will
describe how to use the degeneration formula to get new relations in the cohomology of the
moduli space of complex structures on a genus *g* Riemann surface with
*n* marked points.

Oct. 20, 1999

Paul Feehan (Ohio State)

Witten's conjecture and gluing *PU*(2) monopoles

We will discuss new results on gluing theory for *PU*(2) monopoles and applications
to the proof of Witten's conjecture on the relation between the Donaldson and Seiberg-Witten
invariants of smooth four-manifolds. The *PU*(2)-monopole moduli space provides a
noncompact cobordism between links of compact moduli spaces of *U*(1) monopoles of
Seiberg-Witten type and the (Donaldson) moduli space of anti-self-dual *SO*(3) connections,
which appear as singularities in this larger moduli space. The purpose of the gluing theorem
is to provide topological models for neighborhoods of ideal Seiberg-Witten moduli spaces
appearing in lower levels of the Uhlenbeck compactification of the moduli space of *PU*(2)
monopoles and thus permit calculations of their contributions to Donaldson invariants using
the *PU*(2) monopole cobordism.

Oct. 11, 1999

Moshe Jarden (Tel Aviv)

Torsion on abelian varieties over large algebraic fields

A conjecture of Geyer and the speaker from 1978 asserts finiteness and infiniteness of the
torsion part of abelian varieties over large algebraic fields. A theorem of Geyer and the
speaker says the conjecture holds for elliptic curve. The aim of the talk is to prove
several parts of the conjecture for arbitrary abelian varieties.

Oct. 11, 1999

Ching-Li Chai (Penn)

Congruences of Néron models: solution to a problem of Gross and Prasad

The formation of Néron models of a torus *T* over a local field *K*
does not commute with change of base field; the discrepency gives an numerical invariant
*c*(*T*). For an abelian variety *A* over *K* one can define this
invariant *c*(*A*) similarly, and *c*(*A*) is the local contribution
of the change of Faltings' height when *A* is defined over a global field. In joint
work with J.-K. Yu, we give an affirmative answer to the question of Gross and Prasad,
that *c*(*T*) is equal to one-half of the Artin conductor of the Galois
representation on the character group *X**T*) of *T*. This
is proved directly when *K* has characteristic zero, and the case when *K* has
characteristic *p*>0 is deduced from the characteristic zero case, from a newly
observed phenomenon: congruence of the Galois representation on *X**T*)
implies congruence of the Néron models; this phenomenon also holds for abelian
varieties. This idea goes back to Deligne-Kazhdan-Krasner. There is a different proof of
the question of Gross and Prasad, due to E. de Shalit and based on common germ of idea,
which deals directly with the Lie algebras of the Néron models.

Oct. 6, 1999

Gary Kennedy (UMich visitor)

Contact formulas for rational plane curves

Two algebraic plane curves are said to have a contact of order *n* at a
point if both curves are smooth there and if the intersection number is *n*.
If *X* is a family of such curves depending on s parameters, then one expects
to find finitely many members of the family satisfying s specified conditions, where
a "condition" means either passing through a specified point or having a contact of
order *n* with a specified curve (this should be counted as *n*-1 conditions).
A contact formula expresses this finite number in terms of simpler data for the family
and for the specified individual curves, called characteristic numbers. One is then
faced with the problem of determining the characteristic numbers. We will look in
particular at the case of rational plane curves and where the specified order of
contact is no more than three.

Sept. 29, 1999

Vangelis Mouroukos (UMich)

Arithmetic Hodge structures and higher Abel-Jacobi maps, after Green, Asakura and Saito

One of the traditional tools for the Hodge-theoretic study of algebraic cycles has been
the Abel-Jacobi map, classically for divisors on Riemann surfaces and generalized by Weil
and Griffiths to higher codimension cycles on varieties of arbitrary dimension. However,
both the image and the kernel of the Abel-Jacobi map remain, in general, rather
mysterious. Recent work by various authors associates to any complex variety a new
Hodge-theoretic structure, called an arithmetic Hodge structure. Using these, one can
define higher Abel-Jacobi maps from (higher) Chow groups to certain extension groups of
arithmetic Hodge structures. These new invariants can be used to study the kernel and
image of the Abel-Jacobi map in several interesting situations (for example, in the cases
of zero-cycles on a surface and cycles on general hypersurfaces). We shall give an
exposition of the above constructions and examples to illustrate that the new invariants
can detect non-trivial algebraic cycles quite efficiently.

Sept. 22, 1999

Ana Bravo (UMich visitor)

Smoothness and arithmetical schemes

We study the notion of quasi-smoothness introduced by O. Villamayor in some recent works.
This notion makes possible to associate a "canonical tangent bundle" to certain
arithmetical schemes. In this context we can associate a "jacobian ideal" to any
irreducible scheme embedded in a quasi-smooth scheme. We describe the class of centers so
that quasi-smoothness is preserved by blowing-ups. Finally we characterize this class of
centers in terms of the jacobian ideal.

Apr. 21, 1999

Alexander Barvinok (UMich)

Quadratic convexity

Let *f* : **R**^{n}→**R**^{2} be a map given by a
pair of quadratic forms. A (strong) version of the Toeplitz-Hausdorff Theorem asserts that the
image *f* (*S*) of the unit sphere *S* in **R**^{n}
is a convex set in **R**^{2} provided *n* >2. A similar result holds
for a map **C**^{n}→**R**^{3} defined by three Hermitian forms.
In this talk, I discuss convexity properties of the image for more than two real (three Hermitian)
forms. New convexity results will be presented as well as connections with distance geometry and
molecular conformation.

Apr. 7, 1999

Vangelis Mouroukos (Chicago)

Cohomological connectivity and algebraic cycles

We give a systematic way to produce interesting algebraic cycles on general complete
intersections of sufficiently high multidegree in a non-singular projective variety, that
contain a fixed non-singular subvariety. This gives rise to new non-torsion elements in
the Griffiths group of homologically trivial cycles modulo algebraic equivalence of such
a complete intersection. Moreover, we formulate injectivity and surjectivity results for
the corresponding Chow groups; they are consistent with Beilinson's conjectures on mixed
motives and filtrations on Chow groups.

Apr. 6, 1999

Dennis Gaitsgory

Geometric construction of central elements in the affine Hecke algebra

Let *G* be a *p*-adic group and let *H*_{I} denote the
Iwahori Hecke algebra of *G*, i.e., *H*_{I} consists of compactly
supported functions on *G*, which are bi-invariant with respect to the Iwahori
subgroup of *G*. It is well-known that the center of *H*_{I} is
isomorphic to the spherical Hecke algebra *H*_{s} of *G*, and the
latter can be identified by means of the Satake isomorphism with the Grothendieck ring of
finite-dimensional representations of the corresponding Langlands' dual group. Moreover,
D. Kazhdan and G. Lusztig gave a geometric interpretation of *H*_{I}.
They showed that elements of *H*_{I} can be viewed as perverse sheaves
on a certain infinite-dimensional algebraic variety, called the affine flag manifold.
Therefore, it becomes a natural question to ask if one can construct explicitly the center
of *H*_{I} using algebraic geometry. (In this form the question was
first formulated by R. Kottwitz). In this talk we shall present such a construction, using
Drinfeld's idea of fusion of Hecke operators. Moreover, we shall see that it has many
favorable properties. In particular, it has been used by R. Bezrukavnikov for the proof of
Lusztig's conjecture about the relation between two-sided cells in the affine Weyl group
and centralizers of unipotent elements in the Langlands' dual group. Results which are
going to be mentioned in this talk were obtained in a joint work with A. Beilinson.

Mar. 31, 1999

Matthew Emerton (UMich)

The geometric Langlands construction

In this talk we will explain Beilinson and Drinfeld's construction of the geometric
Langlands correspondence between certain *D*-modules on the moduli of
*G*-bundles on a curve and opers for the dual group on the curve itself.

Mar. 24, 1999

Ravi Vakil (MIT)

Twelve points on the projective line

There are many ways to define a (*PGL*(2)-invariant) divisor in the variety of
"12 points in **P**^{1}", *Sym*^{12}(**P**^{1}),
isomorphic to **P**^{12}. For example: the 12 nodal cubics in a pencil of
plane cubics; the branch points of a degree 4 map of a genus 3 hyperelliptic curve to
**P**^{1}; the branch points of a genus 3 curve mapped to **P**^{1}
by the canonical divisor; the branch points of a genus 4 curve mapped to
**P**^{1} by a theta-divisor. We'll see that these divisors (and others) are
all the same, by showing that the corresponding moduli spaces are covers of others. The
links involve various beautiful classical constructions.

Mar. 23, 1999

Bernd Sturmfels (Berkeley)

Resonant hypergeometric series

We present a basis of logarithmic series solutions at a point of maximal degeneracy to the
Gelfand-Kapranov-Zelevinsky hypergeometric equations. These differential equations were
studied by Batyrev, Hosono-Lian-Yau and Stienstra in the context of toric mirror symmetry.
Our new construction is combinatorial and gives an explicit formula for the terms of such
a series. Main ingredients are volumes of convex polytopes and shellings of
triangulations. This talk is based on the material in Section 3.6 of the forthcoming book
"Gröbner Deformations of Hypergeometric Differential Equations" (with M. Saito and
N. Takayama). The current draft of our book is available at
my homepage.

Mar. 10, 1999

Kalle Karu (Boston U.)

Semistable reduction in characteristic zero

Let *f*: *X*→*B* be a family of varieties. We consider the problem of
replacing the family *f* with a new family *f*': *X*'→*B*' such
that all fibers of *f*' are as nice as possible. The correct definition of "as nice
as possible" is given in terms of toric geometry, and a morphism *f*' satisfying it is
called semistable. The semistable reduction problem then asks to find a generically finite
proper base change *B*'→*B* and a proper birational morphism
*X*'→ *X* x_{B} *B*' such that the induced morphism
*f*': *X*'→*B*' is semsitable. We discuss the proof of a slightly
weaker version of the problem and some interesting (unsolved) questions in the
combinatorics of polyhedral complexes that the strong semistable reduction problem gives
rise to.

Feb. 23 and Mar. 9, 1999

Anders Buch (Chicago)

Formulas for degeneracy loci

I will describe a new class of polynomials which describe the cohomology class of a very
general type of degeneracy loci, associated to a sequence of vector bundle maps and
arbitrary rank conditions on these maps and their compositions. These polynomials
generalize and give new formulas for all known types of Schubert polynomials. Our
polynomials are expressed as a linear combination of products of Schur polynomials.
We conjecture that all coefficients are non-negative, and given by a generalized
Littlewood-Richardson rule. This is joint work with William Fulton.

Feb. 22, 1999

David Ben-Zvi (Harvard)

Spectral curves, opers and integrable systems

We describe some relations between loop groups, soliton equations, and algebraic geometry.
Soliton equations (such as the Korteweg-deVries hierarchy) are naturally expressed as
flows on spaces of flat connections (differential data). On the other hand, integrable
systems in algebraic geometry are often expressed, using the theory of spectral curves,
as linear flows on Jacobians (spectral data). We present a simple and general equivalence
between moduli of differential data and moduli of spectral data, which provides a
comprehensive geometric picture for a wide range of systems. An interesting feature is
the role played by formal versions of spectral curves, which are also responsible for
some interesting phenomena in loop groups.

Feb. 10, 1999

Sándor Kovács (Chicago)

Rational singularities

A new singularity class is introduced in arbitrary characteristic. In characteristic 0
these singularities are shown to be equivalent to rational singularities. As a corollary
we obtain a very simple amd natural proof of Elkik's Theorem: log terminal singularities
are rational. Connections with other flavors of rationality will also be discussed.