The author has worked for 4 years (1993 1996) at the University of Sicgcn in the group of Applied and Numerical Mathematics. His stay was supported by a DFG scholarship and by a guest professorship from the University of Sicgcn. During this time, this lecture was held for graduate students.The group of Applied and Numerical Math, and the students of his lecture like to thank Priv. Doz. Dr. Dinh Nho Hao for his great efforts in research and teaching.
