1. 1 Department of Mathematics and Computer Science , Larbi Tebessi University , Tebessa , Algeria
2. 2 Department of Applied Mathematics , University of Salamanca , Casas del Parque, 2, 37008-Salamanca , Spain
3. 3 SIDIS Research Group, Department of Mathematics, Institute of Applied Mathematics in Science and Engineering (IMACI), Polytechnic School of Cuenca , University of Castilla-La Mancha , Cuenca , Spain