1. Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail:
2. Institute of Mathematics and Mechanics of NAS of Azerbaijan, 9, F. Agaev St., Baku, Az1141, Azerbaijan. E-mail:
3. Department of Mathematics, University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal. E-mail: