1. Riyadh University for Women, Science Section (Mathematics), P. O. Box 60561, Riyadh 11555, Saudi Arabia
2. Instituto de Matemáticas, Universidad Nacional Autónoma de México, C. P. 58089, Morelia, Michoacán, Mexico
3. Mathematics Institute, University of Warwick, Coventry, CV4 7AL, UK
4. Mathematical Institute, University of Debrecen and the Number Theory Research Group of the Hungarian Academy of Sciences, P. O. Box 12, 4010 Debrecen, Hungary