On the algebraic structure of differentially homogeneous polynomials

The paper describes the algebraic structure of the graded algebra of differentially homogeneous polynomials of fixed finite order. We show that it is a finitely generated algebra, and we exhibit a minimal set of generators. Along the way, we provide a simpler proof of the so-called Schmidt--Kolchin conjecture (proved in a previous paper) . From the algebraic point of view, this provides natural compactifications of jet bundles of projective spaces. From the invariant theoretic point of view, this provides new examples, not covered (to our knowledge) by known conjectures in the subject, of unipotent sub-groups of general linear groups, whose algebras of invariants are finitely generated (and more precisely gives a First Fundamental Theorem for such groups, following the terminology in Invariant Theory).