Valencia-Cabrera, L., Orellana-Martín D., Martínez-del-Amor M. A., Riscos-Núñez A., & Pérez-Jiménez M. J.  (2016).??Polarizationless P Systems with Active Membranes: Computational Complexity Aspects.  Journal of Automata, Languages and Combinatorics. 21(1-2),?107?123.