1. Pedagogical Department E.E., Section of Mathematics and Informatics, National and Capodistrian University of Athens, 4, Agamemnonos Str., Aghia Paraskevi, Athens 15342, Greece
2. Department of Mathematics, Thiruvalluvar University College of Arts and Science, Kariyampatti, Tirupattur, 635 901 Tamil Nadu, India
3. Department of Mathematics, Iran University of Science and Technology, Tehran 13114-16846, Iran
4. Institute of Research and Development of Processes IIDP, University of the Basque Country, Campus of Leioa, Spain