Department of Mathematics, University of Oslo, Oslo, Norway
Lab 1, No 1337, Sandvika, Norway and Kleve 4541 m, University of Oslo, Oslo, Norway
Copyright © 2015 Yngvar Reichelt, Karl L. Reichelt et al. This is
an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.