1. Departamento de Matematica Aplicada, Facultad de Ciencias, Universidad de Salamanca, E37008 Salamanca, Spain
2. Section of Mathematics, Department of Civil Engineering, School of Engineering, Democritus University of Thrace, GR-671 00 Xanthi, Greece
3. Departamento de Matemáticas, Facultad de Ciencias, Universidad de Salamanca, E37008 Salamanca, Spain