1. Escuela de Ciencias Físico-Matemáticas, de la Universidad Michoacana, de San Nicolás de Hidalgo, Mexico
2. School of IT and Engineering, University of Ottawa, 800 King Eduard, Ottawa, Ontario, Canada, K1N 6N5, Canada
3. School of Computer Science, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6, Canada
4. Department of Computer Science, Concordia University, 1455 de Maisonneuve Blvd West, Montral, Qubec, Canada, H3G 1M8, Canada
5. Department of Mathematics, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada, V5A 1S6, Canada
6. Instituto de Matemticas, Universidad Nacional Autnoma de Mxico, Circuito Exterior, Ciudad Universitaria, Coyoacn 04510, Mxico, D.F. Mxico, Mexico