The variational complex of a diffeomorphisms group
  In this paper we propose a variational complex associated to a diffeomorphisms group with first order jet in a Lie group. We study the structure of null lagrangians and we prove some fundamental properties of them, as well as their connection to differential invariants of the group action. Further informations at http://irmi.epfl.ch/cag/buliga_necv.html . 
Marius Buliga