A. Alhazov, R. Freund, A. Riscos-Núñez:
"Membrane division, restricted membrane creation and object complexity in P systems";
International Journal of Computer Mathematics, 83 (2006), 7; 529 - 548.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC06587129