1. Department of Mathematics, Universität Innsbruck, Technikerstrasse 13, 6020 Innsbruck, Austria.
2. Department of Mathematics, University of Kurdistan, PO Box 416, Sanandaj, Iran, and Department of Engineering, University of Borås, SE-501 90 Borås, Sweden.
3. Department of Mathematics, University of Kurdistan, PO Box 416, Sanandaj, Iran.