1. Department of Applied Sciences, University of Québec at Chicoutimi, 555, Boul. de l'Université, Chicoutimi, Québec G7H 2B1, Canada
2. Department of Applied Mechanics, Budapest University of Technology and Economics, Budapest H-1521, Hungary
3. Department of Mathematical Sciences, The University of Texas at Dallas, P. O. Box 830688, M/S EC 35, Richardson, TX 75083-0688, USA