@Inproceedings{EPTCS3.9,
  author    = {Freund, Rudolf and Klein, Andreas and Kutrib, Martin},
  year      = {2009},
  title     = {On the Number of Membranes in Unary P Systems},
  editor    = {Dassow, J\"urgen and Pighizzini, Giovanni and Truthe, Bianca},
  booktitle = {{\rm Proceedings Eleventh International Workshop on}
               Descriptional Complexity of Formal Systems,
               {\rm Magdeburg, Germany, July 6-9, 2009}},
  series    = {Electronic Proceedings in Theoretical Computer Science},
  volume    = {3},
  publisher = {Open Publishing Association},
  pages     = {101-109},
  doi       = {10.4204/EPTCS.3.9},
}