1. Department of Mathematics, University of Patras, Patras, Greece
2. Department of Civil Engineering, University of Thessaly, Volos, Greece
3. Technological Educational Institute of Western Greece, Department of Accounting and Finance, Messolonghi, Greece
4. Departamento de Análisis Matemático, Facultad de Matemáticas, Universidad de Santiago de Compostela, Spain
5. Departamento de Psiquiatría Radiología y Salud Pública, Facultad de Medicina, Universidad de Santiago de Compostela, Spain