Exterior Differential System for Cosmological G2 Perfect Fluids and   Geodesic Completeness
  In this paper a new formalism based on exterior differential systems is derived for perfect-fluid spacetimes endowed with an abelian orthogonally transitive G2 group of motions acting on spacelike surfaces. This formulation allows simplifications of Einstein equations and it can be applied for different purposes. As an example a singularity-free metric is rederived in this framework. A sufficient condition for a diagonal metric to be geodesically complete is also provided. 
L. Fernandez-Jambrina, L. M. Gonzalez-Romero