  Scattering resonances have important applications in many areas of science
and engineering. They are the replacement of discrete spectral data for
problems on non-compact domains. In this paper, we consider the computation of
scattering resonances defined on the exterior to a compact sound hard obstacle.
The resonances are the eigenvalues of a holomorphic Fredholm operator function.
We truncate the unbounded domain and impose the Dirichlet-to-Neumann (DtN)
mapping. The problem is then discretized using the linear Lagrange element.
Convergence of the resonances is proved using the abstract approximation theory
for holomorphic Fredholm operator functions. The discretization leads to
nonlinear algebraic eigenvalue problems, which are solved by the recently
developed parallel spectral indicator methods. Numerical examples are presented
for validation.
