Two-source dispersers for polylogarithmic entropy and improved ramsey graphs

Abstract

In his influential 1947 paper that inaugurated the probabilistic method, Erdős proved the existence of 2logn-Ramsey graphs on n vertices. Matching Erdős’ result with a constructive proof is considered a central problem in combinatorics, that has gained a significant attention in the literature. The state of the art result was obtained in the celebrated paper by Barak, Rao, Shaltiel, and Wigderson who constructed a 22(loglogn)1−α-Ramsey graph, for some small universal constant α > 0. In this work, we significantly improve this result and construct 2(loglogn)c-Ramsey graphs, for some universal constant c. In the language of theoretical computer science, this resolves the problem of explicitly constructing dispersers for two n-bit sources with entropy (n). In fact, our disperser is a zero-error disperser that outputs a constant fraction of the entropy. Prior to this work, such dispersers could only support entropy Ω(n).