Abstract
Systems of language equations used by Ginsburg and Rice (“Two families of languages related to ALGOL”, JACM, 1962) to represent context-free grammars are modified to use the symmetric difference operation instead of union. Contrary to a natural expectation that these two types of equations should have incomparable expressive power, it is shown that equations with symmetric difference can express every recursive set by their unique solutions, every recursively enumerable set by their least solutions and every co-recursively-enumerable set by their greatest solutions. The solution existence problem is Π1-complete, the existence of a unique, a least or a greatest solution is Π2-complete, while the existence of finitely many solutions is Σ3-complete.
Supported by the Academy of Finland under grant 206039.
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Okhotin, A. (2006). Language Equations with Symmetric Difference. In: Grigoriev, D., Harrison, J., Hirsch, E.A. (eds) Computer Science – Theory and Applications. CSR 2006. Lecture Notes in Computer Science, vol 3967. Springer, Berlin, Heidelberg. https://doi.org/10.1007/11753728_30
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DOI: https://doi.org/10.1007/11753728_30
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