Institution: | 1. Laboratoire de Mathématiques , Université de Sidi Bel Abbès , BP 89, 22000 Sidi Bel Abbès, Algérie;2. Faculty of Mathematics and Computer Sciences , Nicholas Copernicus University , Chopina 12/18, 87-100 Torun, Poland;3. Department of Mathematics , University of Ioannina , 451 10 Ioannina, Greece |