Picard-Vessiot Extensions of Real Differential Fields

Abstract

For a linear differential equation defined over a formally real differential field K with real closed field of constants k, Crespo, Hajto, and van der Put proved that there exists a unique formally real Picard-Vessiot extension up to K-differential automorphism. However, such an equation may have Picard-Vessiot extensions that are not formally real fields. The differential Galois group of a Picard-Vessiot extension for this equation has the structure of a linear algebraic group defined over k and is a k-form of the differential Galois group H of the equation over the differential field K(√-1). These facts lead us to consider two issues: determining the number of K-differential isomorphism classes of Picard-Vessiot extensions and describing the variation of the differential Galois group in the set of k-forms of H. We address these two issues in the cases when H is a special linear, a special orthogonal, or a symplectic linear algebraic group and conclude that there is no general behaviour.