A basic fact of algebraic graph theory is that the number of connected components in an undirected graph is equal to the multiplicity of the eigenvalue zero in the Laplacian matrix of the graph. In particular, the graph is disconnected if and only if there are at least two eigenvalues equal to zero. Cheeger's inequality and its variants provide an approximate version of the latter fact; they state that a graph has a sparse (non-expanding) cut if and only if there are at least two eigenvalues that are close to zero.

I will talk about two natural notions of convergence for sequences of graphs of bounded degree and their connection to groups and group actions. The first is Benjamini-Schramm convergence, which is strongly related to parameter testing. The second is local-global convergence, introduced by Hatami, Lovasz and Szegedy. I will discuss recent results on roots of graph polynomials (including the chromatic polynomial and Tutte polynomials), the combinatorics of expander graphings, and the geometry of locally symmetric spaces.

The polynomial Freiman-Ruzsa conjecture is one of the important open problems in additive combinatorics. In computer science, it already has several diverse applications: explicit constructions of two-source extractors; improved bounds for the log rank conjecture in communication complexity; and lower bounds for locally decodable codes based on matching vectors codes. Recently, Tom Sanders proved a quasi-polynomial version of this conjecture. I will describe the polynomial Freiman-Ruzsa conjecture, its applications and the proof of Sanders theorem.

In FT-mollification, one smooths a function while maintaining good quantitative control on high-order derivatives. This is a continuation of my talk from last week, and I will continue to describe this approach and show how it can be used to show that bounded independence fools polynomial threshold functions over various distributions (Gaussian, Bernoulli, and p-stable). I may also touch on other applications in approximation theory and learning theory.

This talk is based on various works by subsets of Ilias Diakonikolas, Daniel Kane, David Woodruff, and myself.

The braid group on n strands may be viewed as an infinite analog of the symmetric group on n elements with additional topological phenomena. It appears in several areas of mathematics, physics and computer sciences, including knot theory, algebraic geometry, quantum mechanics, quantum computing and cryptography.

In FT-mollification, one smooths a function while maintaining good quantitative control on high-order derivatives. I will describe this approach and show how it can be used to show that bounded independence fools polynomial threshold functions over various distributions (Gaussian, Bernoulli, and p-stable).

This talk is based on various works by subsets of Ilias Diakonikolas, Daniel Kane, David Woodruff, and myself.

Complexity theory, with some notable exceptions, typically studies the complexity of computing a function h(x) of a *given* input x. We advocate the study of the complexity of generating -- or sampling -- the output distribution h(x) for random x, given random bits.

In particular, we present first-of-their-kind lower bounds for generating distributions in various restricted computational models. We also discuss connections to succinct data structures and to randomness extractors.