Symmetric hyperbolic form of systems of second-order evolution equations   subject to constraints
  Motivated by the initial-boundary value problem for the Einstein equations, we propose a definition of symmetric hyperbolicity for systems of evolution equations that are first order in time but second order in space. This can be used to impose constraint-preserving boundary conditions. The general methods are illustrated in detail in the toy model of electromagnetism. 
Carsten Gundlach, Jose M. Martin-Garcia