Skew derivations with central invariants

If sigma is an automorphism and delta is a sigma-derivation of a ring R, th en the subring of invariants is the set R-(delta) = {r is an element of R \ delta(r)= 0}. The main result of this paper is 'let R be a semiprime ring with an algebraic sigma-derivation delta such that R-(delta) is central; th en R is commutative'. This theorem generalizes results on the invariants of automorphisms and derivations and is proved by reducing down to the specia l cases of automorphisms and derivations.