Department of Mathematics Faculty of Science Universiti Brunei Darussalam Gadong BE 1410 BRUNEI DARUSSALAM
Haydar Akça
Mathematical Sciences Department Faculty of Sciences United Arab Emirates University P. O. Box 17551 Al Ain UAE
Valéry Covachev
Department of Mathematics & Statistics College of Science, P. O. Box 36 Sultan Qaboos University, Muscat 123 SULTANATE OF OMAN Institute of Mathematics Bulgarian Academy of Sciences Sofia BULGARIA