Rees algebras on smooth schemes: integral closure and higher differential operator. O. Villamayor U. (2008) Revista Matemática Iberoamericana
Regular differential forms and duality for projective morphisms. Ernst Kunz, Reinhold Hübl (1990) Journal für die reine und angewandte Mathematik
Rings of differential operators over rational affine curves Gail Letzter, Leonid Makar-Limanov (1990) Bulletin de la Société Mathématique de France