On the occasion of the creation of a table of modular elliptic curves over Q(sqrt(5)), we review the "Remarks on isogenies" that accompanied the "Antwerp" tables (LNM 476), and outline some of the new phenomena and open questions that arise in attempting to give a similar overview of isogenies defined over Q(sqrt(5)) or other number fields. In particular, we account for some new isogeny degrees and graphs not seen over Q, and explain why the problem of proving completeness of the list over Q(sqrt(5)) is difficult but not hopeless.
The existence of a Landau-Siegel zero leads to the Deuring-Heilbronn phenomenon, here appearing in the 1-level density in a family of quadratic twists of a fixed genus character L-function. We obtain explicit lower order terms describing the vertical distribution of the zeros, and realize the influence of the Landau-Siegel zero as a resonance phenomenon.
For any convergent sequence of Riemannian spaces, it is
possible to extract a subsequence for which their corresponding
tangent bundles converge as well. These limits sometimes coincide
with preexisting notions of tangency, but not always. In the process
of understanding the structure of the limiting space, a couple of
natural elementary constructions are introduced at the level of
the individual Riemannian spaces. Lastly, a weak notion of parallelism is
discussed for the limits.
What is the "simplest" knot in a given three-manifold Y?
We know that the answer is the unknot when Y=S^3, as the unknot
happens to be the only knot in the three-sphere with the smallest
genus (=0). In this talk, we will discuss the more general notion of
the rational genus of knots. In particular, we will show that the
simple knots are really the "simplest" knots in the lens spaces in
the sense of being a genus minimizer in its homology class. This is
a joint work with Yi Ni.
We continue with thermodynamic formalism from last time, this time applying what we have learned to hyperbolic dynamical systems. In particular, we shall see how thermodynamic formalism can be applied to obtain information about fractal dimensions of hyperbolic sets (which is, in a sense, a measure of complexity of the system). In particular, we shall cover: the Bowen's equation (relating topological pressure to Hausdorff dimension), which is a very broad generalization of Moran's theorem for some iterated function systems, as well as Ruelle's theorem (asserting that, in some sense, only observables at periodic points are needed to completely determine Bowen's equation). If time permits, we shall describe an approach, using thermodynamic formalism, to one of the open problems in spectral theory of quasiperiodic Schroedinger operators, that was presented by Anton Gorodetski at the seminar on Oct. 16th.
Thermodynamic formalism as a mathematical theory has its roots in one of the most successful theories of physics -- thermodynamics and statistical mechanics. In its core, the theory of thermodynamic formalism seeks to describe properties of observable "macroscopic" phenomena based on the average behavior of the "microscopic" constituents. In the language of dynamical systems: given a dynamical system $(X, f)$, with $X$ the phase space and $f$ the map defining the dynamics, one seeks to describe properties of functions defined on $X$ (the macroscopic observables) based on the (often averaged, in some well-defined sense) behavior of $f$. In particular, thermodynamic formalism leads to strong results in dimension theory of dynamical systems (e.g. describing fractal dimensions and measures of sets arising as invariant sets of some chaotic dynamical systems). In this first of a series of two talks, we shall present the main ingredients of thermodynamic formalism: topological entropy, metric entropy, topological pressure, and the variational principle for the pressure.
As, beginning with the famous Hofstadter's butterfly, all
numerical studies of spectral and dynamical quantities related to
quasiperiodic operators are actually performed for their rational
frequency approximants, the questions of continuity upon such
approximation are of fundamental importance. The fact that continuity
issues may be delicate is illustrated by the recently discovered
discontinuity of the Lyapunov exponent for non-analytic potentials.
I will review the subject and then focus on work in progress, joint with Avila and Sadel, where we develop a new approach to continuity, powerful enough to handle matrices of any size and leading to a number of strong consequences.
This is the first of (likely) two talks, where an almost entire proof will be presented. For understanding most of the talk knowledge of spectral theory should not be necessary and just knowing some basic harmonic analysis should suffice.