1. Center for Theoretical Chemistry and Physics, New Zealand Institute for Advanced Study, Massey University Albany, Auckland 0745, New Zealand
2. Department of Mathematics, Eastern Mediterranean University, Famagusta, TRNC, Mersin 10, Turkey
3. Department of Mathematical Sciences, University of Agder, Serviceboks 422, 4604 Kristiansand, Norway
4. Department of Mathematics and Mathematical Statistics, Umeå University, 90187 Umeå, Sweden