Prolongation of Tanaka structures: an alternative approach
  The classical theory of prolongation of G-structures was generalized by N. Tanaka to a wide class of geometric structures (Tanaka structures), which are defined on a non-holonomic distribution. Examples of Tanaka structures include subriemannian, subconformal, CR-structures, structures associated to second order differential equations and structures defined by gradings of Lie algebras (in the setting of parabolic geometry). Tanaka's prolongation procedure associates to a Tanaka structure of finite order a manifold with an absolute parallelism. It is a very fruitful method for the description of local invariants, investigation of the automorphism group and the equivalence problem. In this paper we develop an alternative constructive approach for Tanaka's prolongation procedure, based on the theory of quasi-gradations in filtered vector spaces, G-structures and their torsion functions. 
Dmitri V. Alekseevsky, Liana David
math.DG