Picard-Vessiot extensions, linear differential algebraic groups and their torsors

Abstract

Let K be differential field with algebraically closed field of constants. Let K^diff be a differential closure of K, and L the (iterated) Picard-Vessiot closure of K inside K^diff. Let G be a linear differential algebraic group over K and X a differential algebraic torsor for G over K. We prove that X(L) is Kolchin-dense in X. When G is finite-dimensional we prove that X(L) = X(K^diff). We give close relationships between Picard-Vessiot extensions of K and torsors for suitable finite-dimensional linear differential algebraic groups over K. We suggest some differential field analogues of the notion of boundedness for fields (Serre's property F).