1. Department of Applied Mathematics and HCM, BiBoS, IZKS, University of Bonn, Germany
2. CERFIM, Locarno, Switzerland. E-mail: albeverio@uni-bonn.de
3. Department of Mathematics and Statistics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. E-mail: boubaker@kfupm.edu.sa