# School of Mathematics

## On Bilinear Complexity

For a set of polynomials F, we define their bilinear complexity as the smallest k so that F lies in an ideal generated by k bilinear polynomials. The main open problem is to estimate the bilinear complexity of the single polynomial $\sum_{i,j}x_i^2 y_j^2$. This question is related to the classical sum-of-squares problem as well as to problems in arithmetic circuit complexity. We will focus on related sets of polynomials and prove some lower and upper bounds on their bilinear complexity.

## The SOS (aka Lassere/Positivestellensatz/Sum-of-Squares) System Series

We will give an overview of this system, which has been at the center of recent algorithmic and proof complexity developments. We will give the definitions of the system (as a proof system for polynomial inequalities, and as an SDP-based algorithm), and basic upper and lower bounds for it. In particular we'll explain the recent SOS-proof of the hypercontractive inequality for the noisy hypercube of Barak et al., as well as the degree lower bounds for proving Tseitin and Knapsack tautologies of Grigoriev.

## Invariance Under Isomorphism and Definability

## Local Global Principles for Galois Cohomology

We consider Galois cohomology groups over function fields F of curves that are defined over a complete discretely valued field.

Motivated by work of Kato and others for n=3, we show that local-global principles hold for

$H^n(F, Z/mZ(n-1))$ for all n>1.

In the case n=1, a local-global principle need not hold. Instead, we will see that the obstruction to a local-global principle for $H^1(F,G)$, a Tate-Shafarevich set, can be described explicitly for many (not necessarily abelian) linear algebraic groups G.

## Universality in Mean Curvature Flow Neckpinches

This is from joint works with D. Knopf and I. M. Sigal. In this talk I will present a new strategy in studying neckpinching of mean curvature flow. Different from previous results, we do not use backward heat kernel, entropy estimates or subsequent convergence, instead we apply almost precise estimates, invented in the past few years, to obtain the first result on asymmetric surface.

## Working Group on Univalent Foundations

## Combinatorial PCPs with Short Proofs

The PCP theorem (Arora et. al., J. ACM 45(1,3)) asserts the existence of proofs that can be verified by reading a very small part of the proof. Since the discovery of the theorem, there has been a considerable work on improving the theorem in terms of the length of the proofs, culminating in the construction of PCPs of quasi-linear length, by Ben-Sasson and Sudan (SICOMP 38(2)) and Dinur (J. ACM 54(3)).

## Beauty in Mathematics

Often mathematicians refer to a "beautiful" result or a "beautiful" proof. In this special lecture, Enrico Bombieri, Professor Emeritus in the School of Mathematics, addresses the question, "What is beauty in mathematics?"

## Quantum Beauty

Does the world embody beautiful ideas? This is a question that people have thought about for a long time. Pythagoras and Plato intuited that the world should embody beautiful ideas; Newton and Maxwell demonstrated how the world could embody beautiful ideas, in specific impressive cases. Finally in the twentieth century in modern physics, and especially in quantum physics, we find a definitive answer: Yes! The world does embody beautiful ideas.