  Generalizing the main result of [Aparicio-Monforte A., Compoint E., Weil
J.-A., J. Pure Appl. Algebra 217 (2013), 1504-1516], we prove that a linear
differential system is in reduced form in the sense of Kolchin and Kovacic if
and only if any differential module in an algebraic construction admits a
constant basis. Then we derive an explicit version of this statement. We
finally deduce some properties of the Lie algebra of Katz's intrinsic Galois
group.
