Key points are not available for this paper at this time.
Einstein's equations are not a well-posed system of evolution equations for the spatial metric, except in special coordinates. A remarkable first-order symmetrizable hyperbolic formulation is found that is surprisingly close to Einstein's original equations yet does not require such coordinates. This system has only physical characteristic directions, the light cone and the normal to the spacelike foliation, and serves to unify all the physical hyperbolic formulations.
Anderson et al. (1999) studied this question.