  The Generalized Finite Element Method (GFEM) is an extension of the Finite
Element Method (FEM), where the standard finite element space is augmented with
a space of non-polynomial functions, called the enrichment space. The functions
in the enrichment space mimic the local behavior of the unknown solution of the
underlying variational problem. GFEM has been successfully applied to a wide
range of problems. However, it often suffers from bad conditioning, i.e., its
conditioning may not be robust with respect to the mesh and in fact, the
conditioning could be much worse than that of the standard FEM. In this paper,
we present a numerical study that shows that if the "angle" between the finite
element space and the enrichment space is bounded away from 0, uniformly with
respect to the mesh, then the GFEM is stable, i.e., the conditioning of GFEM is
not worse than that of the standard FEM. A GFEM with this property is called a
Stable GFEM (SGFEM). The last part of the paper is devoted to the derivation of
a robust iterative solver exploiting this angle condition. It is shown that the
required "wall-clock" time is greatly reduced compared to popular GFEMs used in
the literature.
