# Undecidable problems concerning densities of languages

Abstract : In this paper we prove that the question whether a language presented by a context free grammar has density, is undecidable. Moreover we show that there is no algorithm which, given two unambiguous context free grammars on input, decides whether the language defined by the first grammar has a relative density in the language defined by the second one. Our techniques can be extended to show that this problem is undecidable even for languages given by grammars from $LL(k)$ (for sufficiently large fixed $k ∈ \mathbb{N} )$.
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Communication dans un congrès
David, René and Gardy, Danièle and Lescanne, Pierre and Zaionc, Marek. Computational Logic and Applications, CLA '05, 2005, Chambéry, France. Discrete Mathematics and Theoretical Computer Science, DMTCS Proceedings vol. AF, Computational Logic and Applications (CLA '05), pp.69-76, 2005, DMTCS Proceedings
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https://hal.inria.fr/hal-01183335
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• HAL Id : hal-01183335, version 1

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Jakub Kozik. Undecidable problems concerning densities of languages. David, René and Gardy, Danièle and Lescanne, Pierre and Zaionc, Marek. Computational Logic and Applications, CLA '05, 2005, Chambéry, France. Discrete Mathematics and Theoretical Computer Science, DMTCS Proceedings vol. AF, Computational Logic and Applications (CLA '05), pp.69-76, 2005, DMTCS Proceedings. 〈hal-01183335〉

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