Differential Galois Theory and Hopf Algebras for Lie Pseudogroups

Abstract

According to a quite clever but never acknowledged work of E. Vessiot that won the prize of the Acad\'{e}mie des Sciences in 1904, " Differential Galois Theory " (DGT) has mainly to do with the study of " Principal Homogeneous Spaces " (PHS) for finite groups ( classical Galois theory), algebraic groups (Picard-Vessiot theory) and algebraic pseudogroups (Drach-Vessiot theory). The corresponding automorphic differential extension are such that $ dim_K(L)< \infty $, transcendence degree $ trd(L/K)< \infty $ and $ trd(L/K)=\infty $ with $ diff trd(L/K)< \infty $ respectively. The purpose of this paper is to mix differential algebra, differential geometry and algebraic geometry in order to revisit DGT, pointing out the deep confusion between {\it prime differential ideals} ( Defined by J.-F. Ritt in 1930) and {\it maximal ideals} thad has been spoiling the works of Vessiot, Drach, Kolchin and all followers. In particular, we use Hopf algebras in order to study the structure of the algebraic Lie pseudogroups involved, namely Lie pseudogroups defined by systems of algebraic OD or PD equations. Many explicit examples are presented for the first time in order to illustrate these results. This paper is also paying a tribute to Prof. A. Bialynicki-Birula on the occasion of his recent death in April 2021 at the age of 90 years old. His main idea has been to notice that an algebraic group $G$ acting on itself is the simplest example of a PHS. If $G$ is defined over a field $K$ and we introduce the algebraic extension $L=K(G)$, then there is a Galois correspondence between the intermediate fields $K \subset K' \subset L$ and the subgroups $e \subset G' \subset G $ provided that $K'$ is stable under a Lie algebra $\Delta $ of invariant derivations of $L/K$. Our purpose is to extend this result from algebraic groups to algebraic pseudogroups without {\it any} way to use parameters.