Hopf dense Galois extensions with applications

Let H be a finite dimensional Hopf algebra, and let A be a left H-module algebra. Motivated by the study of the isolated singularities of AH and the endomorphism ring EndAH(A), we introduce the concept of Hopf dense Galois extensions in this paper. Hopf dense Galois extensions yield certain equivalences between the quotient categories over A and AH. A special class of Hopf dense Galois extensions consists of the so-called densely group graded algebras, which are weaker versions of strongly graded algebras. A weaker version of Dade's Theorem holds for densely group graded algebras. As applications, we recover the classical equivalence of the noncommutative projective scheme over a noetherian N-graded algebra A and its d-th Veronese subalgebra A(d) respectively. Hopf dense Galois extensions are also applied to the study of noncommutative graded isolated singularities.