1. Lect Notes Comput Sci;L. Cardelli,1998
2. J. Castellanos, A. Rodriguez-Paton, Gh. Pąun, Computing with membranes: P systems with worm-objects, IEEE 7th. Intern. Conf. on String Processing and Information Retrieval, SPIRE 2000, La Coruna, Spain, 64–74.
3. J. Dassow, Gh. Pąun, Regulated Rewriting in Formal Language Theory, Springer-Verlag, Berlin, 1989.
4. M. R. Garey, D. S. Johnson, Computers and intractability. A guide to the theory of NP-completeness, 1979, W. H. Freeman and Company.
5. D. Hauschild, M. Jantzen, Petri nets algorithms in the theory of matrix grammars, Acta Informatica, 31 (1994), 719–728.