1. Research Center for Astronomy and Applied Mathematics, Academy of Athens, Soranou Efesiou 4, Athens GR-11527, Greece
2. School of Mathematics, University of Bristol, Fry Building, Woodland Road, Bristol BS8 1UG, UK
3. Department of Mathematics, United States Naval Academy, Chauvenet Hall, 572C Holloway Road, Annapolis, MD 21402-5002, USA