1. Faculty of Mechanics and Mathematics, Taras Shevchenko National University of Kyiv, 64 Volodymyrska St., Kyiv 01033, Ukraine. Tel.: +380 44 259 05 40; Fax: +380 44 259 07 85; E-mail: melnyk@imath.kiev.ua
2. Department of Mathematics, Swansea University, Swansea SA2 8PP, UK