1. Simion Stoilow Institute of Mathematics of the Romanian Academy, P.O. Box 1 – 764, RO – 010145 Bucharest 1, Romania.
2. Bursa Uludağ University, Department of Mathematics, 16059 Bursa, Turkey.
3. Hacettepe University, Department of Mathematics, Beytepe Campus 06532 Ankara, Turkey.