... Langer1
Institute for Analysis and Numerical Mathematics, Johannes Kepler University Linz, Altenberger Str. 69, A - 4040 Linz, Austria. e-mail: kuhn@numa.uni-linz.ac.at.      Acknowledgment: This work is supported by the Austrian Science Fund - 'Fonds zur Förderung der wissenschaftlichen Forschung' - under project P 11215-TEC.
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