Work on these articles have been supported by the National Science Foundation since 1990, including the following grants: DMS-1200271, DMS09-01163, DSM04-00482, DMS01-00441, DMS98-00609, DSM95-02186,

- An algebraic construction
of an abelian variety with a given Weil number
(with Frans Oort) We give an algebraic proof of the existence of
a CM abelian variety with a given CM type, and deduce from it
the existence part of the Honda-Tate theorem, without using
complex uniformization of abelian varieties.

- Complex Multiplication and
Lifting Problems
(with Brian Conrad and Frans Oort),
Mathematical Surveys and Monographs, volume 195, American Mathematical
Society, 2014, 387 + ix pp, ISBN 978-1-4704-1014-8.

There are three main results in this book on CM lifting problems for abelian varieties and p-divisible groups.

(1) a necessary and sufficient condition for the existence of a CM lifting for an abelian variety over a finite field with a given CM structure to a characteristic zero normal domain (chapter 2),

(2) an obstruction for the existence of CM liftings for a p-divisible group, extending Oort's abelian variety example (chapter 3),

(3) existence of a CM lifting for an an abelian variety with a given CM structure over a finite field to a characteristic 0 domain (chapter 4).

Other materials in the book include:

(4) a review of basic CM theory (chapter 1),

(5) a "modern style" proof of the main theorem of complex multiplication and a converse (appendix A),

(6) existence of algebraic Hecke characters with a given algebraic part over the field of moduli of the algebraic part which has good reduction over a given finite set of finite places, existence of CM p-divisible groups with a given p-adic CM type over the reflex field (appendix A),

(7) alternative proofs of the existence of CM lifting using p-adic Hodge theory (appendix B).

- Abelian varieties isogenous
to a Jacobian (with Frans Oort),
Annals of Math. 176 (2012), 589-635. Under either the GRH or
the Andre-Oort conjecture, we show that for every g at least 4,
there exists an abelian variety over the field of algebraic numbers
which is NOT isogenous to a Jacobian. Furthermore, there
are at most a finite number curves of genus g whose Jacobian
has complex multiplication by a CM field of degree 2g whose
Galois group has maximal order, 2^g g!.
We also establish the generalization to Shimura varieties.

- Correction to "A note on Manin's
theorem of the kernel", Amer. J. Math. 113, 1991, 387-389.

- Local monodromy of Hilbert
modular varieties. We prove a constancy result of the local monodromy
of a Hilbert modular variety on the zero locus of Hasse invariants,
and prove that they are maximal.

- Mumford's example of non-flat
Pic^{\tau}. In Seminaire Bourbaki 1961/62, no. 236, Grothendieck
remarked that Mumford had an example of a non-flat Pic^{\tau}, and said
that it is a deformation of an Igusa surface over an artin ring.
We work out such an example.

- Monodromy and irreducibility
of leaves (.pdf file) (with Frans Oort) We prove that for the
moduli space of principally polarized abelian varieties,
the non-supersingular Newton polygon strata and leaves are irreducible,
and their p-adic monodromy are maximal. Annals of Math. 173 (2011),
1359-1396.

- Moduli of abelian varieties
and p-divisible groups: Density of Hecke orbits, and a conjecture
of Grothendieck (.pdf file) Expository article based on notes for a
Conference on Arithmetic Geometry, Goettingen, July 17 - August 11,
2006. In "Arithmetic Algebraic Geometry", Clay Mathematics Proceedings 8,
2009 (Darmon, Ellwood, Hassett, Tschinkel eds.), 441-536.

- Methods for p-adic monodromy
(.pdf file)
We explain three methods for showing that the p-adic
monodromy attached to a modular family of abelian varieties is
"as large as possible". J. Inst. Math. 7 (2008), 247-268.
published version

- Hecke orbits and
irreducibility of leaves(with Frans Oort)
Notes for a talk at the Workshop on abelian varieties, Amsterdam,
May 29-May 31, 2006.

- Hecke orbits as Shimura
varieties in positive characteristic.
Proceeding of ICM2006 Madrid, a survey areticle on the Hecke orbit
problem. Some errors in the published version are corrected here.

- Hypersymmetric
abelian varieties, .pdf file of published version
(Joint with Frans Oort) We explain the notation of hypersymmetric
abelian varieties. Included are the existence of simple hypersymmetric
abelian varieties over a finite field with a given Newton polygon
satisfying a suitable condition (called "balanced"), as well as
a partial converse to the Honda-Tate theorem.
This notation was motivated by the Hecke orbit problem.
Pure Appl. Math. Quaterly 2 (Coates special issue), 2006, 1-27.

- Canonical coordinates on
leaves of p-divisible groups: The two-slope case, .pdf file
The formal completion at any point of a central leaf in a modular
variety of PEL-type is built-up from p-divisible formal groups
by a family of successive fibrations. In this article we treat
the essential case where the Barsotti-Tate group has exactly two
slopes. Then the formal completion at a point of a leaf is
the maximal p-divislble subgroup of the "extension part" of
the local deformation space. A "triple Cartier module", defined
to be the set of all p-typical curves of the Cartier ring functor,
plays an important role.

- Hecke orbits on Siegel
modular varieties, .pdf file
This is a survey article on the Hecke orbit conjecture for
Siegel modular varieties. We sketch a proof of the conjecture and
describe the theories developed for the conjecture.
Progress in Math. 235, Birkhauser, 2004, pp. 71-107.

- Families of ordinary abelian
varieties: canonical coordinates, p-adic monodromy, Tate-linear
subvarieties and Hecke orbits, .pdf file
This paper was motivated by the ordinary case of the Hecke orbit
problem. Methods were developed along several related threads.
Several conjectures were formulated, including an analog of the
Mumford-Tate conjecture for ordinary abelian varieties.
These conjectures hold in the case of Hilbert modular varieties.

- Hypersymmetric abelian
varieties, .pdf file
This is an older version, introducing the notion of
"hypersymmetric abelian varieties":
their ring of endomorphism up to isogeny is as large as
the slope condition allowed. They play a useful role in the
proof of the Hecke orbit conjecture for Siegel modular varieties.
The new version
is joint with Frans Oort.

- Monodromy of Hecke-invariant
subvarieties, .pdf file of published version, or
Monodromy of Hecke-invariant
subvarieties, .dvi file.
We use a group-theoretic argument to show that for any
l-adic monodromy group attached to the Zariski closure of
the Hecke orbit of a non-supersingular point in the moduli
space of principally polarized abelian varieties of characteristic
p is equal to the full symplectic group.
Pure Appl. Math. Quarterly 1 (Borel special issue), 2005, 291-303.

- A rigidity result
for
p-divisible formal groups, .pdf file
We prove a generalization of the following statement.
Let Z be an irreducible closed formal subvariety of
a formal torus T, and suppose that Z is stable under
multiplication by 1+p^n for some integer n>1.
Then Z is a formal subtorus of T. Asian J. Math. 12 (2008), 193-202.

- Elementary divisors of the
base change conductor for tori, .pdf file, or
Elementary divisors of the
base change conductor for tori, .dvi file.
This note contains some estimates for the "elementary divisors"
of the base change conductor for tori over local fields.
One can view these "elementary divisors" as numerical invariants
of local Galois representations, with values in a general linear
group with coefficients in (p-adic) integers.
The estimates in this note are definitely not sharp, except perhaps
the first and the last one among the elementary divisors.

- A bisection of the Artin conductor,
.pdf file,
or
A bisection of the Artin conductor,
.dvi file. In this paper we give a formula for the base change
conductor of an abelian variety over a local field with potentially
ordinary reduction.

- Neron modesl for semiabelian
varieties: congruence and change of base field, .ps file,
or
Neron modesl for semiabelian
varieties: congruence and change of base field, .pdf file

(Asian Journal of Math. vol. 4, No. 4, 715-736, December 2000.)

We study an invariant, called the "base change conductor", for semiabelian varieties over local fields. The case for tori is dealt with in the joint paper with J.-K. Yu and E. de Shalit.

- Congruences of Neron Models for
Tori and the Artin Conductors, .dvi file or
Congruences of Neron Models for
Tori and the Artin Conductors, .ps file

(Joint work with J.-K. Yu and E. de Shalit, Annals of Math. 154, 2001, 347-382) We prove that the Neron models of two tori are congruent if their Galois representation on the character groups are sufficiently congruent. The point is that the two tori may be defined over local fields with different characteristics. The generic characteristic zero is easier to analyse, and the case of positive characteristic is "reduced" to that.

- A Note on the Existence of
Absolutely Simple Jacobians, .dvi file or
A Note on the Existence of
Absolutely Simple Jacobians, .ps file

(Joint work with Frans Oort, Jour. Pure Appl. Alg. vol. 155, 2001, 115-120.)

We show that the subset of curves of genus g over finite fields whose Jacobians are absolutely simple has positive density in the moduli space of curves.

- Character Sums, Automorphic
Forms, Equidistribution, and Ramanujan Graphs, part1, .dvi file,
Character Sums, Automorphic
Forms, Equidistribution, and Ramanujan Graphs, part1, .pdf file;
and
Character Sums, Automorphic
Forms, Equidistribution, and Ramanujan Graphs, part2, .dvi file,
Character Sums, Automorphic
Forms, Equidistribution, and Ramanujan Graphs, part2, .pdf file.
Joint work with Wen-Ching Winnie Li. The main tools used
are the machinery of l-adic cohomology and the converse theorem for
automorphic representations.

- Geometry of Shimura Varieties in
Positive Characteristics, .dvi file or
Geometry of Shimura Varieties in
Positive Characteristics, .ps file
Slides of a talk at ICCM 1998, Beijing.

- Newton Polygons as Lattice
Points, .dvi file or
Newton Polygons as Lattice
Points, .ps file

(Amer. J. Math. 122, 2000, 967-990.)

A combinatorial study of Newton points for*F*-isocrystals with additional structures, cast against the background of roots and weights. In connection with Shimura varieties, there is a formula expressing the predicted dimension of the Newton strata of the reduction of a Shimura variety.

- Density of Hecke Orbits
for Abelian Varieties of
*p*-corank one, .dvi file or Density of Hecke Orbits for Abelian Varieties of*p*-corank one, .ps file Any such symplectic isogeny class in the moduli space of*g*-dimensional principally polarized abelian varieties in characteristic*p*is dense.

- Local Monodromy for Deformations of
One-Dimensional Formal Groups, .dvi file or
Local Monodromy for Deformations of
One-Dimensional Formal Groups, .ps file

(J. reine angew. Math. 524, 2000, 227-238.)

Over an equal-characteristic*p*formal power series ring, if a one-dimensional formal group has a closed fiber with height*h*and generic fiber with height*1*, then the*(h-1)*-dimensional*p*-adic Galois representation is irreducible.

- Density of Members
with Extra Hodge Cycles in a Family of Hodge Structures, .dvi file
or Density of Members
with Extra Hodge Cycles in a Family of Hodge Structures, .ps file
of the galley
You will find an easily computable invariant
*c*such that the set of all such members is dense if the codimension of the image of the period map is at most*c*and the period space is a hermitian symmetric domain.

- The Naturality in
Kirwan's Decomposition, .dvi file or
The Naturality in
Kirwan's Decomposition, .ps file

This is a short joint paper with Amnon Neeman. Please contact us if you can either prove or disprove the conjectures here.

- Every Ordinary Symplectic
Isogeny Class in Characteristic
*p*is Dense in the Moduli, .pdf file of published version, Every Ordinary Symplectic Isogeny Class in Characteristic*p*is Dense in the Moduli, .dvi file or Every Ordinary Symplectic Isogeny Class in Characteristic*p*is Dense in the Moduli, .ps file

The main result is already stated in the title. Please let me know if you find an application of the main theorem here.

- The Group Action on the Closed
Fiber of the Lubin-Tate Moduli Space, .dvi file or
The Group Action on the Closed
Fiber of the Lubin-Tate Moduli Space, .ps file

Motivated and written before the main result of the previous paper was proved, this whole paper is a long-and-dirty calculation of the*initial terms*of the action of the stabilizer subgroup in the Lubin-Tate case. (This group is also known as the Morava stabilizer subgroup in stable homotopy theory.) This is one of the few cases where high-order deformations are calculated, and may be of interest only to those who like to make further progress in understanding this action.