Abstract
In this paper we generalize Posner's first theorem to a 3-prime near-ring with a (sigma, tau)-derivation. We prove that a prime ring with a non-zero (sigma, tau)-derivation is commutative if sigma(x)d(x) = d(x)tau(x) for all x is an element of U where U is a suitable subset of R. Also, we generalize Posner's second theorem completely to a prime ring with a (sigma, sigma)-derivation and partially to a prime ring with a (sigma, tau)-derivation.