[1] Sahand Univ Technol, Fac Basic Sci, Dept Math, Tabriz, Iran
[2] Islamic Azad Univ, Dept Math, Lahijan Branch, Lahijan, Iran
[3] Univ Porto, Fac Engn, UISPA, LAETA INEGI, Rua Dr Roberto Frias, P-4200465 Porto, Portugal
[4] Inst Engn, Dept Elect Engn, Rua Dr Antonio Bernardino de Almeida, P-4249015 Porto, Portugal