1. INSA de Rouen Normandie Normandie University LMI 76000 Rouen France
2. Research Center for Pure and App. Math., Graduate School of Information Sciences Tohoku University Japan
3. School of Mathematics and Statistics Shandong University, Weihai Weihai 264209 China
4. LAMA, Univ Gustave Eiffel, UPEM Univ Paris Est Creteil CNRS F‐77447 Marne‐la‐Vallée France
5. Faculty of Mathematics Al. I. Cuza University Bd. Carol I, 11 Iasi 700506 Romania
6. O. Mayer Institute of Mathematics Romanian Academy Bd. Carol I, 8 Iasi 700505 Romania