1. University of Porto, Rua Dr. Roberto Frias,Research Center for Systems and Technologies (SYSTEC), Faculty of Engineering,Porto,Portugal,4200-465
2. School of Sciences, University of Padova,Department of Mathematics “Tullio Levi-Civita”,Via Trieste,Padova,Italy,63 - 35121
3. University of Eduardo Mondlane,Faculty of Sciences, Av. Julius Nyerere, nr.,Department of Mathematics and Informatics,Maputo,Mozambique,3453