R. Freund, M. Oswald:
"P Systems with Activated/Prohibited Membrane Channels";
in: "Membrane Computing. International Workshop, WMC-CdeA 2002, Curtea de Arges, Romania", Gh. Paun et al. (ed.); Springer, 2003, ISBN: 3-540-00611-7, 261 - 268.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04404612