Toward the history of dynamical entropy: comparing two definitions

Two definitions are compared for the measure-theoretic entropy of an automorphism of a Lebesgue space: the commonly known definition suggested in 1959 by Ya. G. Sinai and the definition from the unpublished master thesis by D. Z. Arov (1957). The result is that the two definitions lead to essentially the same object in the class of ergodic automorphisms, while for the nonergodic case the situation is in general different.