  We identify a class of time-periodic linear symmetric hyperbolic equations
that are finite codimension stable, because an associated operator has compact
resolvent, sufficiently far to the right in the complex plane. This paper is an
attempt to capture abstractly the observation in numerical general relativity
that some discretely self-similar spacetimes, such as Choptuik's critical
spacetime, are finite codimension stable.

Analysis of PDEs;General Relativity and Quantum Cosmology