1. Department of Mathematics and Informatics, “Gh. Asachi” Technical University of Iaşi Bd. Carol I, No. 11 RO-700506 Iaşi, Romania
2. Institute of Mathematics University, of Tsukuba 1-1-1 Tennodai Tsukuba, Ibaraki 350-0006, Japan
3. Faculty of Mathematics, “AL. I. Cuza” University of Iaşi Bd. Carol I, No. 11 RO-700506 Iaşi, Romania