1. Simion Stoilow Institute of Mathematics of the Romanian Academy, Research Unit 5, P.O. Box 1 – 764, RO – 010145 Bucharest 1, Romania
2. Department of Mathematics, Bucharest University, Academiei Str. 14, RO – 010014 Bucharest 1, Romania
3. Department of Mathematics, Hacettepe University, Beytepe Campus 06532 Ankara, Turkey