Projective ideals of skew polynomial rings over HNP rings

Let R be an hereditary Noetherian prime ring (or, HNP-ring, for short), and let S = R[x; sigma] be a skew polynomial ring over R with sigma being an automorphism on R. The aim of the paper is to describe completely the structure of right projective ideals of R[x; sigma] where R is an HNP-ring and to obtain that any right projective ideal of S is of the form Xb[x; sigma], where X is an invertible ideal of S and b is a sigma-invariant eventually idempotent ideal of R.