The determinant bound for discrepancy is almost tight

Abstract

In 1986 Lovasz, Spencer, and Vesztergombi proved a lower bound for the hereditary a discrepancy of a set system F in terms of determinants of square submatrices of the incidence matrix of F. As shown by an example of Hoffman, this bound can differ from herdisc(F) by a multiplicative factor of order almost log n, where n is the size of the ground set of F. We prove that it never differs by more than O((log n)3/2), assuming |F| bounded by a polynomial in n. We also prove that if such an F is the union of t systems F_1, . . ., F_t, each of hereditary discrepancy at most D, then herdisc(F) \leq O(t^(1/2)(log n)^(3/2) D). For t = 2, this almost answers a question of Sos. The proof is based on a recent algorithmic result of Bansal, which computes low-discrepancy colorings using semidefinite programming.