On the discrepancy of random low degree set systems

Abstract

Motivated by the celebrated Beck-Fiala conjecture, we consider the random setting where there are n elements and m sets and each element lies in t randomly chosen sets. In this setting, Ezra and Lovett showed an O((tlog t)1/2) discrepancy bound in the regime when n ≤ m and an O(1) bound when n mt. In this paper, we give a tight O(t) bound for the entire range of n and m, under a mild assumption that t = Ω(log log m)2. The result is based on two steps. First, applying the partial coloring method to the case when n = mlogO(1)m and using the properties of the random set system we show that the overall discrepancy incurred is at most O(t). Second, we reduce the general case to that of n ≤ mlogO(1)m using LP duality and a careful counting argument.