@article {, title = {An approach to computational complexity in Membrane Computing}, journal = {Brainstorming Workshop on Uncertainty in Membrane Computing}, year = {2004}, month = {November 8-10}, pages = {67-94}, publisher = {Department de Matem{\`a}tiques i Inform{\`a}tica de la Universitat de les Illes Balears}, address = {Palma de Mallorca}, isbn = {84-7632-897-4}, author = {Mario J. P{\'e}rez-Jim{\'e}nez} }