 The approach is global in character, and differs from the functional analysis techniques traditionally used to study differential operators.The strongest results are obtained for over-determined systems (holonomic systems), and on the characteristic variety cut out by the symbols, in the good case for which it is a Lagrangian submanifold of the cotangent bundle of maximal dimension (involutive systems).The techniques were taken up from the side of the Grothendieck school by Zoghman Mebkhout, who obtained a general, derived category version of the Riemann–Hilbert correspondence in all dimensions.