Second-Order Bounds on Correlations Between Increasing Families

Abstract

Harris’s correlation inequality states that any two monotone functions on the Boolean hy-percube are positively correlated. Talagrand [11] started a line of works in search of quantitative versions of this fact by providing a lower bound on the correlation in terms of the influences of the functions. A famous conjecture of Chvátal [2] was found by Friedgut, Kahn, Kalai and Keller [5] to be equivalent to a certain strengthening of Talagrand’s bound, conjectured to hold true when one of the functions is antipodal (i.e., g(x) = 1 − g(−x). Motivated by this conjecture, we strengthen some of those bounds by giving estimates that also involve the second order Fourier coefficients of the functions. In particular, we show that the bounds due to Talagrand and due to Keller, Mossel and Sen [8] can be improved when one of the functions is antipodal. Our proofs follow a different route than the ones in the literature, and the analysis is carried out in the Gaussian setting.