U.C. Santa Cruz, Department of Applied Mathematics and Statistics, School of Engineering University of California, Santa Cruz 1156, Santa Cruz, CA 95064 United States
Universidad Sim¢n Bol¡var, Universidad Sim¢n Bol¡var Departamento de C¢mputo Cient¡fico y Estad¡stica Apartado postal 89.000, Caracas, 1080-A Venezuela ;