R. Freund, A. Paun:
"P Systems with Active Membranes and without Polarizations";
in: "Second Brainstorming Week on Membrane Computing", Gh. Paun, A. Riscos-Núñez, A. Romero-Jiménez, F. Sancho-Caparrini (ed.); Universidad de Sevilla, Sevilla, 2004, 193 - 205.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04968494