
THEMATIC PROGRAMS 

September 19, 2017  
Automorphic Forms ProgramWorkshop on Shimura varieties and related topics

James Arthur, University of Toronto
On the universal groups in automorphic forms and algebraic geometry
It is believed that there are universal groups that govern fundamental processes in number theory. One of these is the Langlands group, which is conjectured to classify automorphic representations. Another is the motivic Galois group of Grothendieck, which is thought to classify parallel objects in algebraic geometry. We shall discuss some questions on the possible structure of these groups.
Don Blasius,UCLA
Weightmonodromy conjecture for some simple Shimura varieties
We will prove Ramanujan Conjecture and WeightMonodromy conjecture for Shimura varieties of arbitrary dimension attached to forms of GU(2) and GU(3).
ChingLi Chai, University of Pennsylvania
The Hecke orbit problem: Hecke symmetries and Oort foliation
One striking feature of the moduli space $\mathcal{A}_g$ is that it has a lot of symmetries, usually attributed to Hecke. These symmetries are algebraic correspondences on $\mathcal{A}_g$ coming from the symplectic group $GSp(2g)$. Over the complex numbers these Hecke correspondences are usually studied using the complex uniformization of $\mathcal{A}_g$ described above. Over a field of positive characteristic, for instance the algebraic closure of $\mathbb{Z}/p\mathbb{Z}$, the moduli space $\mathcal{A}_g$ has fine structures not coming from characteristic \(0\), because the local structure of abelian varieties in characteristic \(p\) may not all ``look alike''. Typical examples of such fine structures include the Newton polygon stratification and the EkedahlOort stratification. These two fine structures are both preserved by the Hecke symmetries, but in general they are not enough to characterize the orbit of the Hecke symmetries. \smallbreak
Recently Oort defined a ``foliation'' structure on the moduli space
$\mathcal{A}_g$ in characteristic \(p\). Conjecturally the Zariski closure
of the orbits of the Hecke symmetries are exactly the closures of ``leaves''
in Oort's foliation. We will explain some recent progress on the Hecke
orbit problem, including the local structure of the leaves. The latter
generalizes a theorem of Serre and Tate announced in the 1964 Summer
Institute of Algebraic Geometry at Woods Hole.
Laurent Fargues, Universite de Paris
VII (Jussieu)
Boundary cohomology of LubinTate spaces
I will explain how a generalized theory of canonical subgroups applies to exhibit a stratification of LubinTate spaces outside sufficiently great compact subset of the open padic ball. To each of this stratum is associated a parabolic subgroup of $GL_n$ and the cohomology (in the limit) of this stratum is parabolicaly induced. This applies to a conjecture of Prasad on autodual representations of division algebras. I will discuss how this should apply to finish M. Strauch program on proving part of Harris Taylor results by purely local methods.
Jens Funke, Fields Institute
(Singular) theta lifts and the construction of Green currents
and differential characters for cycles in locally symmetric spaces of
orthogonal and unitary type
In this talk, we establish a duality result between the singular theta
lift introduced by Borcherds and the KudlaMillson theta lift. For O(p,2),
this gives rise to the construction of Green functions for certain special
cycles, while for arbitrary signature, one obtains differential
characters in the sense of Cheeger and Simons. We also discuss the extension
of these results to unitary groups.
This is joint work with Jan Bruinier.
Ulrich Goertz, Universitaet Koeln
The JordanHoelder series of nearby cycle sheaves on some
Shimura varieties and affine flag varieties
We consider sheaves of nearby cycles on certain Shimura varieties with Iwahori level structure and affine flag varieties, respectively. Describing the JordanHoelder series of these sheaves reveals interesting combinatorial patterns, which can be interpreted in terms of the cohomology of certain intersections of Schubert cells and opposite Schubert cells in the affine flag variety for the corresponding group. This is joint work with Thomas Haines.
Eyal Goren, McGill University
Local models and displays
I would like to start with some basics about local models  in fact with no level p involved  to give some examples and illustrate some phenomena. I would then like to explain what are displays, how one uses them to study deformation problems and a technical lemma that allows one to calculate universal displays. This will then be applied to calculating the universal display of Hilbert modular varieties in the case of bad reduction, and time permitting, in cases of quaternion algebras ramified at p.
Haruzo Hida, UCLA
Automorphism Groups of Shimura Varieties mod $p$
We try to prove that any scheme automorphism of each geometrically irreducible component of a certain Shimura variety of PEL type modulo $p$ (of primeto$p$ level) is given by an isogeny action (generalizing a result of Shimura in characteristic 0). This can be generalized also to the naive Igusa tower over the variety. If time allows, we would be able to discuss some application of this result.
Tetsushi Ito, MaxPlanck Institut (Bonn)
Weightmonodromy conjecture for padically uniformized varieties
In this talk, I will give a proof of the weightmonodromy conjecture (Deligne's conjecture on the purity of monodromy filtration) for varieties with padic uniformization by the Drinfeld upper half spaces of any dimension. The ingredients of the proof are to prove a special case of the Hodge standard conjecture, and apply an argument of Steenbrink, M. Saito to the weight spectral sequence of RapoportZink. As an application, by combining this result with the results of SchneiderStuhler, we compute the local zeta functions of padically uniformized varieties in terms of representation theoretic invariants.
Gerard Laumon, Universite de ParisSud
(Orsay)
On the fundamental lemma for unitary groups
The ``Fundamental Lemma'' is a series of combinatorial identities which have been discovered by Langlands and which have been precisely formulated by Langlands and Shelstad. Over a non archimedian local field of equal characteristics, the orbital integrals which enter in the statement of the ``Fundamental Lemma'' for unitary groups are directly related to the affine Springer fibers for the general linear groups and it is not difficult to formulate a geometric conjecture which implies the ``Fundamental Lemma''. In this talk, I would like to explain a natural link between those affine Springer fibers and some compactified Jacobians of singular curves. Moreover, using this link, I would like to show that the above geometric conjecture follows from the purity conjecture for the cohomology of the affine Springer fibers of Goresky, Kottwitz and MacPherson.
Ron Livne, The Hebrew University of Jerusalem
Local points and parity of jacobians
Poonen and Stoll had observed that the CasselsTate pairing on the ShafarevichTate group of a principally polarized abelian variety over a global field need not be alternating. Thus the order of $\Sha$ need not be a square.
For a jacobian of a curve there is an explicit criterion to decide when this happens in terms of the local points at primes of bad reduction. We will apply this to Shimura curves. The analysis of these local points uses the padic uniformization, and can be done in much greater generality.
The work described is joint with B. Jordan and Y. Varshavsky.
Elena Mantovan, Berkeley
On certain unitary group Shimura varieties
We shall describe the geometry of a certain class of PEL type Shimura varieties. In particular, we shall study the Newton polygon stratification of the reduction in positive characteristic p of these Shimura varieties. We shall show that each stratum can be described in terms of the products of some smooth varieties (we call Igusa varieties) with the reduced fibers of the pertinent RapoportZink spaces, and of the action on them of a certain padic group (which depends on the stratum). As a result of this analysis, weobtain a description of the ladic cohomology of the Shimura varieties in terms of the ladic cohomology with compact supports of the Igusa varieties and of the RapoportZink spaces, in the appropriate Grothendieck group.
Sascha Orlik, Universitaet Leipzig
The cohomology of period domains
We compute the \'etale cohomology of period domains over local fields for quasisplit reductive groups. The period domains, which were introduced by Rapoport and Zink, are open admissible rigidanalytic subsets of generalized flag varieties. They parametrize (weakly) admissible filtrations of a given isocrystal with additional structure of a reductive group.
Michael Rapoport, Universitaet Koeln
An update on affine DeligueLuszlig varieties
After defining the objects in my title, I will survey recent results on the due to Kottvitz, Leigh, Reuman and Wintenberger.
Jeremy Teitelbaum, University of
Illinois (Chicago)
Padic analytic representation theory and inklings of a padic analytic
Langlands correspondenc
Is there a "padic analytic Langlands correspondence" which
gives an automorphic interpretation to the families of overconvergent
modular forms and padic Galois representations that have been constructed
by Hida and Coleman? In this talk I will survey some recent developments
suggesting that such a correspondence exists. In particular I will give
an overview of the theory of padic analytic representation theory I
have been developing with Peter Schneider and discuss briefly how it
has been related to arithmetic by Breuil and Emerton.
Torsten Wedhorn, Universitaet Koeln
A generalization of the EkedahlOort stratification
In this joint work with Ben Moonen a generalization of the EkedahlOort
stratification is described: For every abelian scheme over a base scheme
$S$ of characteristic $p$, Ekedahl and Oort defined a stratification
of $S$. This has been in particular succesfully applied to universal
abelian schemes over moduli spaces of abelian varieties. We generalize
this to arbitrary proper and smooth schemes of Hodge type. It turns
out that we get stratifications which are indexed by certain quotients
of a symmetric group. There is also the notion of this stratification
with $G$structure for $G$ a reductive group and using results of Lusztig
we get a stratification indexed by a certain quotient
of the Weyl group of $G$.
Uwe Weselmann, University of Heidelberg
A twisted topological trace formula and liftings of automorphic
representations
For the cohomology groups of locally symmetric spaces attached to a
connected reductive group $G$ we developed a topological trace formula
describing the action of Hecke operators twisted by an outer automorphism
$\eta$. In some cases this twisted trace formula is stable and may be
compared with the untwisted trace formula for the stable endoscopic
group $G_1$. The fundamental lemma for this comparison in the case $G=PGl_5$,
$G_1=Sp_4$ can be shown (joint work with J. Ballmann and R. Weissauer)
to be equivalent to the fundamental lemma in the case $G=GL_4\times
GL_1$ and $G_1=GSp_4=GSpin_5$ (proved by Flicker). Using character identities
between local representations and properties of the $\theta$lift one
gets multiplicity results for automorphic representations contributing
to the cohomology of Siegel modular 3folds.
Tonghai Yang, University of Wisconsin
Doubling integral and a variant of the GrossZagier formula
This is part of joint work with Steve Kudla and Michael Rapoport. We will first describe how to write the Lfunction of a modular form as an doubling integral against some Eisenstein series on Sp(4), following PiateskiShapiro and Rallis. We will then explain how to relate the central derivative of the Eisenstein series with Height pairing of Heegner divisors. Finally we put them together to obtain a variant of the GrossZagier formula, and also a variant of Gross's formula on central Lvalue.
ChaiFu Yu, National TsingHua University
Fine structures and Hecke orbit problems of HilbertBlumenthal
varieties
We will report current progress on the reduction of HilbertBlumenthal varieties. We determine the dimension and singularities of the strata defined by various padit invariants. The invariants include Lie types, anumbers and leaves. We will also explain the connection with the Hecke orbit problems and the irreducibility of the nonsupersingular strata. We can prove the irreducibility of these nonsupersingular strata in many cases, including the cases when p is unramified or totally ramified. For the supersingular strata, the number of irreducible components is determined and is expressed in terms of special values of the zeta function. This is a joint work with ChingLi Chai.