Antonia CHINNI∗, Beatrice Di BELLA†
Department of Engineering, University of MessinaContrada di Dio, S. Agata, 98166 Messina,
ItalyPetru JEBELEAN‡ and C ̆alin S ̧ERBAN§
Department of Mathematics, West University of Timi ̧soara Blvd. V. Pˆarvan, no. 4, 300223 Timi ̧soara, Romania