The main idea of upper bounding (3) is to use Rademachers. A Rademacher variable σ simply takes the two values +1 and −1 each with probability 1/2. Let σ1, . . . , σn be n independent Rademachers that are also independent of X1, . . . , Xn and X ′ 1, . . . , X ′ n. For each i, note that the distribution of f(Xi)−f(X ′ i) is the same as the distribution of f(X ′ i)− f(Xi). Therefore, the distrib...