P-Systems and Context Free Languages

P-systems are computing models, where certain objects can evolve in parallel into a hierarchical membrane structure. These membranes cannot preserve any order between their own objects. So, if we wish to treat their content as a sequence of characters, then any order will be correct. Thus, languages as {α|α ∈ {a, b} + , # a (α) = # b (α)} and {a n b n |n ≥ 1} for example, are considered and treated identical. A construction which preserves the order of the characters in the words for context-free languages is presented in this paper. It is made in P-systems with active membranes, where all evolution rules are context-free. Some preliminary decision problems concerning the words of context-free languages generated with this type of P-systems are considered; namely, it can be checked if there are sequences of a given length, or if two sequences are equal or not.