1. Department of Mathematics, Applied Mathematics and Statistics, Case Western Reserve University, Cleveland, Ohio 44106, USA
2. Department of Mathematics, University of Akron, Akron, Ohio 44325, USA
3. Department of Materials Science and Physical Chemistry, and Institute of Nanoscience and Nanotechnology, University of Barcelona, Barcelona 08028, Spain and Institute of Nanoscience and Nanotechnology, Barcelona 08028, Spain
4. School of Mathematics, University of Minnesota, Minneapolis, Minnesota 55455, USA