1. Department of Mathematics, University of Ioannina, 45110 Ioannina, Greece
2. Department of Applied Mathematics, University of Crete, 71409 Heraklion, Greece
3. Department of Mathematics, University of Crete, 71409 Heraklion, Greece
4. Institute of Applied and Computational Mathematics, FORTH, 71110 Heraklion, Greece