There is an abundance of empirical evidence in the numerical relativity
literature that the form in which the Einstein evolution equations are written
plays a significant role in the lifetime of numerical simulations. This paper
attempts to present a consistent framework for modifying any system of
evolution equations by adding terms that push the evolution toward the
constraint hypersurface. The method is, in principle, applicable to any system
of partial differential equations which can be divided into evolution equations
and constraints, although it is only demonstrated here through an application
to the Maxwell equations.Comment: 6 pages, 3 figures, 1 table. Uses REVTeX