January 9 
NO SEMINAR

January 16 
NO SEMINAR

January 23 
Sorina Ionica (ENS Paris) An isogeny graph is a graph whose vertices are principally polarized
abelian varieties and whose edges are isogenies between these varieties. In
his thesis, Kohel described the structure of isogeny graphs for elliptic
curves and showed that one may compute the endomorphism ring of an elliptic
curve defined over a finite field by using a depth first search algorithm
in the graph. In dimension 2, the structure of isogeny graphs is less understood and existing algorithms for computing endomorphism rings are very expensive.
We fully describe the isogeny graphs between genus 2 jacobians with complex multiplication,
with the assumptions that the real multiplication subring is maximal and
has class number one. We derive a depth first search algorithm for computing endomorphism rings locally at prime numbers,
if the real multiplication is maximal. To the best of our knowledge, this is the first DFSbased algorithm in genus 2. (Joint work with Emmanuel Thomé).

January 30 
NO SEMINAR

February 6 
Daniel Kane (Stanford) Let E/Q be an elliptic curve with full 2torsion over Q. We
wish to study the distribution of the ranks of the 2Selmer groups of
twists of E as we vary the twist parameter. A recent result of
SwinnertonDyer shows that if E has no cyclic 4isogeny defined over
Q, then the density of twists with given rank approaches a particular
distribution. Unfortunately SwinnertonDyer used an unusual notion of
density essentially given as the number of primes dividing the twist
parameter goes to infinity. We extend this result to cover density in
the natural sense. 
February 13 
Ronen Mukamel (Stanford) For each real quadratic order O, there is a Weierstrass curve W
in the Hilbert modular surface parametrizing Jacobians with real
multiplication by O. The curve W emerges from the study of billiards in
polygons and is important in Teichmüller theory because its natural
immersion into the moduli space of curves is isometric. Such an immersion
is called a Teichmüller curve.
We will present explicit algebraic models for Weierstrass curves obtained
by studying Hilbert modular forms. We will also present evidence from our
examples that suggest a rich arithmetic associated to Teichmüller curves. 
February 20 
NO SEMINAR

February 27 
NO SEMINAR

March 6 
David Krumm (Claremont McKenna) Let C be a hyperelliptic curve defined over the rational numbers, and consider the set S of all squarefree integers d such that the quadratic twist of C by d has a rational point. In this talk we will discuss the question of whether, given a prime number p, the set S contains representatives from all congruence classes modulo p. When C has genus 0 this question can be answered using elementary number theory, but for higher genera it seems to require the use of big conjectures in arithmetic geometry.

March 13 
Gunther Cornelissen (Utrecht) I will show how to find uniform finiteness results for certain diophantine equations in terms of the Laplace spectrum of an associated graph. The method is to bound the "gonality" of a curve (minimal degree of a map onto a line) by the "stable gonality" of an associated stable reduction graph, and then to bound this stable gonality of the graph (some kind of minimal degree of a map to a tree) in terms of spectral data. The latter bound is a graph theoretical analogue of a famous inequality of Li and Yau in differential geometry. An example of an application is to bound the degree of the modular parametrisation of elliptic curves over function fields. (Joint work with Fumiharu Kato and Janne Kool.) 
March 20 
NO SEMINAR
