The return-times theorem of Bourgain, Furstenberg, Katznelson, and Ornstein states: Theorem [BFKO]. Given any measure-preserving transformation of a probability space (X, F, mu, T) and f is an element of L-infinity(mu), there is a subset X-0 subset of or equal to X of full measure so that for any x(0) is an element of X-0 and second dynamical system (Y, G, nu, S) and g is an element of L-1(nu),
... [Show full abstract] there is a subset Y-0 = Y-0((Y, G, nu, S, g),((X, F, mu, T, f),x(0))) of full measure so that for any y(0) is an element of Y-0, [GRAPHICS] converges.