  Two-level domain decomposition methods are preconditioned Krylov solvers.
What separates one and two-level domain decomposition method is the presence of
a coarse space in the latter. The abstract Schwarz framework is a formalism
that allows to define and study a large variety of two-level methods. The
objective of this article is to define, in the abstract Schwarz framework, a
family of coarse spaces called the GenEO coarse spaces (for Generalized
Eigenvalues in the Overlaps). This is a generalization of existing methods for
particular choices of domain decomposition methods. Bounds for the condition
numbers of the preconditioned operators are proved that are independent of the
parameters in the problem (e.g., any coefficients in an underlying PDE or the
number of subdomains). The coarse spaces are computed by finding low or high
frequency spaces of some well chosen generalized eigenvalue problems in each
subdomain.
