The rank condition and strong rank conditions for Ore extensions

Let R be a ring, σ: R → R a ring endomorphism, and δ: R → R a σ-derivation. We establish that the Ore extension R[x; σ,δ] satisfies the rank condition if and only if R does. In addition, we prove analogous results for the right and left strong rank conditions. However, in the right case, the "if"part requires the hypothesis that σ is an automorphism, whereas, in the left case, this assumption is needed for the "only if"part. Finally, we provide a new proof of an old result of Susan Montgomery stating that a skew power series ring is directly (respectively, stably) finite if and only if its coefficient ring is directly (respectively, stably) finite.