A. Alhazov, R. Freund, Y. Rogozhin:
"Some Optimal Results on Symport/Antiport P Systems with Minimal Cooperation";
in: "Cellular Computing (Complexity Aspects)", M.A. Gutiérrez-Naranjo, Gh. Paun, M. Pérez-Jiménez (ed.); Fénix Editora, Sevilla, 2005, 23 - 36.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936350