Department of Mathematics
University of Giessen
Arndtstrasse 2
D-35392 Giessen
Germany
c/o Department of Mathematics
Faculty of Science and Mathematics
University of Nis
Visegradska 33
18000 Nis
Yugoslavia
Vesna Velickovic
Department of Mathematics
Faculty of Science and Mathematics
University of Nis
Visegradska 33
18000 Nis
Yugoslavia
