1. Dipartimento di Matematica e Applicazioni “R. Caccioppoli” Università di Napoli Federico II Complesso Monte S. Angelo, via Cintia Napoli 80126 Italy
2. Laboratory of Mathematics Graduate School of Engineering Hiroshima University Higashihiroshima 739‐8527 Japan
3. Department of Mathematics Osaka City University 3‐3‐138 Sugimoto, Sumiyoshi‐ku Osaka 558‐8585 Japan