1. Department of Mathematics, National Technical University, Zografou Campus, Athens15780, Greece
2. Institute of Mathematics “Simion Stoilow” of the Romanian Academy, P.O. Box 1-764, 014700Bucharest; and Department of Mathematics, University of Craiova, 200585 Craiova, Romania
3. Faculty of Education and Faculty of Mathematics and Physics, University of Ljubljana, SI-1000Ljubljana, Slovenia