1. 1Department of Mathematics, TAMU, College Station, TX 77843-3368, USA.
2. 2Departamento de Matemáticas, Universidad Nacional de Colombia, Carrera 30, calle 45, Ciudad Universitaria, Bogotá, Colombia.
3. 3Institute of Information and Communication Technologies, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 2, 1113 - Sofia, BULGARIA.