Abstract:Systems of equations of the form Xi = ϕi(X1, . . . , Xn), for 1 i n, in which the unknowns Xi are formal languages, and the right-hand sides ϕi may contain concatenation and union, are known for representing context-free grammars. If, instead of union only, another set of Boolean operations is used, the expressive power of such equations may change: for example, using both union and intersection leads to conjunctive grammars [A. Okhotin, J. Automata, Languages and Combinatorics 6 (2001) 519-535], whereas using… Show more
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