1. 1Kent State University, Kent, Ohio 44242, U.S.A.
2. 2Dublin City University, Glasnevin, Dublin 9, Ireland.
3. 3Trinity College, Dublin 2, Ireland.
4. 4Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, 16 S. Kovalevskaya Str., 620219 Ekaterinburg, Russia.