1. Institute of Computer Science of the Romanian Academy, Iasi Branch, 700481 Iași, Romania
2. Laboratoire de Mathématiques et Applications, UMR CNRS 7348, SP2MI, Université de Poitiers, 86962 Poitiers, CEDEX, France
3. Department of Mathematics, “Alexandru Ioan Cuza” University, Bd. Carol I, 11, 700506 Iaşi, Romania