1. Department of Mathematics and Statistics, University of Victoria, Victoria, British Columbia, Canada%SR92-01.48
2. Kocaeli University, Civil Aviation College, Arslanbey Campus, Izmit-Kocaeli, Turkey%SR92-01.11
3. Department of Mathematics, Faculty of Science, Atatürk University, Yakutiye-Erzurum, Turkey%SR92-01.11
4. Department of Mathematics, Faculty of Science and Art, Erzincan University, Erzincan, Turkey%SR92-01.11