R. Freund, M. Oswald, A. Paun:
"P Systems Generating Trees";
in: "Membrane Computing. International Workshop, WMC 2004", G. Mauri, Gh. Paun, M. Pérez-Jiménez, G. Rozenberg, A. Salomaa (ed.); Springer, LNCS, Berlin, 2005, ISBN: 3-540-25080-8, 309 - 319.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936363