Department of Mathematics, University of the Aegean, Karlovassi, Samos, Greece
Department of Mathematics, University of the Aegean, Karlovassi, Samos, Greece
Copyright © 2016 Christos E. Kountzakis, Maria P. Koutsouraki et al. This is
an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.