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A refined Galerkin error and stability analysis for highly indefinite variational problems

## Lehel Banjai (University of Zürich)

Recently, a refined finite element analysis for highly indefinite Helmholtz problems was introduced by the second author.
We generalise the analysis to the Galerkin method applied to an *abstract*, highly indefinite variational problem.
In the refined analysis, the condition for stability and a quasi-optimal error estimate is expressed in terms of
approximation properties *T(S) \approx S* and *T(u+S) \approx S*. Here, *u* is the solution
of the original variational problem, *T* is a certain continuous solution operator, and *S* is the finite
dimensional test and trial space.

The abstract analysis can be applied to both finite and boundary element solutions of high-frequency Helmholtz problems.
We apply the analysis to investigate the properties of the Brakhage-Werner boundary integral formulation of the Helmholtz
problem, discretised by a standard Galerkin boundary element method. In the case of scattering by the unit sphere,
we derive the explicit dependence of the error and of the stability condition on the wave number *k*. We show that
*hk \lesssim 1* is a sufficient condition for stability and a quasi-optimal error estimate. Further, we show that the
constant of quasi-optimality is independent of *k*, which is an improvement over previously available results.
Thus, the boundary element method does not suffer from the *pollution effect*.

This is joint work with S. Sauter, University of Zürich.