  The construction of fast iterative solvers for the indefinite time-harmonic
Maxwell's system at mid- to high-frequency is a problem of great current
interest. Some of the difficulties that arise are similar to those encountered
in the case of the mid- to high-frequency Helmholtz equation. Here we
investigate how two-level domain-decomposition preconditioners recently
proposed for the Helmholtz equation work in the Maxwell case, both from the
theoretical and numerical points of view.
