A sharp version of Talagrand's selector process conjecture, with applications to rounding fractional covers and Bernoulli Sudakov minoration

Abstract

We prove a sharp version of Talagrand's selector process conjecture. Roughly speaking, given any collection of nonnegative weight vectors whose support form a family that is not $p$-small, a random set of density $O(sp)$ captures all but a $2^{-s}$ fraction of weight of some vector with high probability. This gives a common strengthening of Talagrand's selector process conjecture and the Kahn--Kalai conjecture. We give two applications of this result. First, towards a conjecture of Talagrand on the equivalence of expectation thresholds and fractional expectation thresholds, we show that a $p$-small fractional cover supported on sets of size at most $t$ can be rounded to a $cp/\log(2t)$-small integral cover. As a corollary, we show that the fractional and integral expectation thresholds are separated by at most a $\log \log$ factor. Second, we prove a Sudakov minoration principle for general positive selector processes, which in particular resolves a problem of Talagrand on Sudakov minoration for the product Bernoulli measure.