Number Theory

Number theory is concerned with Diophantine equations and their solutions, encoded in discrete structures involving integers, rational numbers or algebraic quantities. Topology studies the properties of shapes that are unchanged under continuous or smooth deformations, a technique of choice being the construction of appropriate homological invariants. It turns out--perhaps surprisingly to the uninitiated--that these invariants can be endowed with sufficient structure to capture a tremendous amount of arithmetic information. The powerful interplay between arithmetic and topological ideas underlies the most important breakthroughs in the study of Diophantine equations, such as Faltings’ proof of the Mordell Conjecture and Wiles’ proof of Fermat’s Last Theorem. It is also at the heart of more recent and still very fragmentary attempts to construct algebraic points on elliptic curves when their existence is predicted by the Birch and Swinnerton-Dyer conjecture. This lecture will attempt to give a non-technical sampler of some of the rich, fascinating interactions between arithmetic questions and topological insights.

I will discuss techniques to get upper and lower bounds for moments of zeta and L-functions. The lower bounds are unconditional and the upper bounds in general rely on the Riemann Hypothesis. In several cases of low moments, one can obtain asymptotics, and I may discuss a couple of such recent cases.

This lecture is part of a series of 3

1. Lecture 1: distribution-values-zeta-and-l-functions-1-3
2. Lecture 2: Moments of zeta and L-functions on the Critical Line, I
3. Lecture 3: Moments of zeta and L-functions on the critical line, II

I will discuss the distribution of values of zeta and L-functions when restricted to the right of the critical line. Here the values are well understood by probabilistic models involving “random Euler products”. This fails on the critical line, and the L-values here have a different flavor here with Selberg’s theorem on log normality being a representative result.

This lecture is part of a series of 3

1. Lecture 1: distribution-values-zeta-and-l-functions-1-3
2. Lecture 2: Moments of zeta and L-functions on the Critical Line, I
3. Lecture 3: Moments of zeta and L-functions on the critical line, II

Artin conjectured that each of his non-abelian L-series extends to an entire function if the associated Galois representation is nontrivial and irreducible. We will discuss the status of this conjecture and discuss briefly its relation to the Langlands program.

This lecture is part of a series of 3.

• Lecture 1: Introduction to Artin L-series
• Lecture 2: Holomorphy Conjecture and Recent Progress
• Lecture 3: Special Values of Artin L-series

This is a lecture given on the occasion of the launch of the PIMS CRG in "L-functions and Number Theory".

The theory of expander graphs is undergoing intensive development. It finds more and more applications to diverse areas of mathematics. In this talk, aimed at a general audience, I will introduce the concept of expander graphs and discuss some interesting connections to arithmetic geometry, group theory and cryptography, including some very recent breakthroughs.