 His paper "On the Time Average Theorem in Dynamics", which appeared in the Proceedings of the National Academy of Sciences, is considered by many to be the first readable paper in modern ergodic theory.His book "Mathematical problems of radiative equilibrium" first appeared in 1934 and was reprinted in 1964.Another important contribution from this period is the theory of Wiener-Hopf equations, which he developed in collaboration with Norbert Wiener.