If #sigma# is an automorphism and #delta# is a #sigma#-derivation of a ring R, then the subring of invariants is the set R~((#delta#)) = {r implied 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; then R is commutative'. This theorem generalizes results on the invariants of automorphisms and derivations and is proved by reducing down to the special cases of automorphisms and derivations.
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