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M. Faustmann, J. Melenk, D. Praetorius:
"Existence of H-matrix approximants to the inverses of BEM matrices: the simple-layer operator";
in: "ASC Report 37/2013", issued by: Institute for Analysis and Scientific Computing; Vienna University of Technology, Wien, 2013, ISBN: 978-3-902627-06-3, 1 - 33.

We consider the question of approximating the inverse W = V−1 of the Galerkin stiffness matrix V obtained by discretizing the simple-layer operator V with piecewise constant functions.
The block partitioning ofWis assumed to satisfy any of the standard admissibility criteria that are employed in connection with clustering algorithms to approximate the discrete BEM operator V.
We show thatW can be approximated by blockwise low-rank matrices such that the error decays exponentially in the block rank employed. Similar exponential approximability results are shown
for the Cholesky factorization of V.

http://www.asc.tuwien.ac.at/preprint/2013/asc37x2013.pdf