1. a Mathematics Department , University of Strathclyde , Livingstone Tower, 26 Richmond Street, Glasgow, Gl 1XH, Scotland
2. b Department of Theoretical Mechanics , University of Nottingham , University Park, Nottingham, NG7 2RD, England
3. c Department of Electrical Engineering , Faculty of Engineering, Nagaoka University of Technology , Kamitomioka 1603-1, Nagaoka, Nilgata, 940-27, Japan