1. Department of Mathematics , Babeş-Bolyai University , M. Kogălniceanu Street, nr. 1, Cluj-Napoca , Romania ; Tiberiu Popoviciu Institute of Numerical Analysis, Romanian Academy , Fântânele Street nr. 57 , Cluj-Napoca , Romania
2. Department of Mathematics , Babeş-Bolyai University , M. Kogălniceanu Street, nr. 1 , Cluj-Napoca , Romania ; Academy of Romanian Scientists Independenţei Street , nr. 54 Bucharest , Romania