Cordón-Franco, A., Gutiérrez-Naranjo M. A., Pérez-Jiménez M. J., & Riscos-Núñez A.  (2004).??Weak metrics on configurations of a P system.  (Gheorghe Paun, Agustín Riscos-Núñez, Álvaro Romero-Jiménez, Fernando Sancho-Caparrini, Ed.).Second Brainstorming Week on Membrane Computing.  139-151.