A. Alhazov, R. Freund, Gh. Paun:
"Computational Completeness of P Systems with Active Membranes and Two Polarizations";
in: "Machines, Computations, and Universality", M. Margenstern (ed.); Springer, LNCS, Berlin, 2005, ISBN: 3-540-25261-4, 82 - 92.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05936359