Institution: | (1) Departamento de Matemáticas, CINVESTAV-IPN, A. Postal 14-740, México D.F. 07000, México. Phone: (52-5) 747-3871. Fax: (52-5) 747-3876. E-mail: (JAM) jalvarez@math.cinvestav.mx, (OHL) ohernand@math.cinvestav.mx, MX;(2) Also at Departamento de Sistemas, UAM-Azcapotzalco, Ave. San Pablo No. 180, México D.F. 02200, México, on sabbatical leave from CINVESTAV-IPN., MX |