1. Faculty of Science, Department of Mathematics, Mansoura University, Mansoura, Egypt
2. Department of Mathematics, University of Cádiz, Puerto Real, España
3. CMUC, Department of Mathematics, University of Coimbra, Coimbra, Portugal
4. Departamento de Álgebra Geometría y Topología, Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos s/n. 29071 Málaga, Spain