[1] Univ Complutense Madrid, Fac Informat, Dept Matemat Aplicada, E-28040 Madrid, Spain
[2] Univ Complutense Madrid, ETSI Telecomunicac, Dept Matemat Aplicada TT 2, E-28040 Madrid, Spain
[3] Univ Cent Venezuela, Fac Ciencias, Escuela Fis, Caracas 1041 A, Venezuela
[4] Univ Simon Bolivar, Dept Fis, Caracas 1080 A, Venezuela