Mixed Elliptic and Hyperbolic Systems for the Einstein Equations
  We analyse the mathematical underpinnings of a mixed hyperbolic-elliptic form of the Einstein equations of motion in which the lapse function is determined by specified mean curvature and the shift is arbitrary. We also examine a new recently-published first order symmetric hyperbolic form of the equations of motion. This paper is dedicated to Andre Lichnerowicz on the occasion of his 80th birthday and will appear in a volume edited by G. Ferrarese. 
Y. Choquet-Bruhat and J.W. York, Jr