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Avoidable Sets and Well Quasi-Orders

  • Flavio D’Alessandro
  • Stefano Varricchio
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 3340)

Abstract

Let I be a finite set of words and \(\Rightarrow_{I}^{*}\) be the derivation relation generated by the set of productions {εu | uI }. Let L I ε be the set of words u such that \(\epsilon {\Rightarrow_{I}^{*}}\). We prove that the set I is unavoidable if and only if the relation \(\Rightarrow_{I}^{*}\) is a well quasi-order on the set L I ε . This result generalizes a theorem of [7]. Further generalizations are investigated.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2004

Authors and Affiliations

  • Flavio D’Alessandro
    • 1
  • Stefano Varricchio
    • 2
  1. 1.Dipartimento di MatematicaUniversità di Roma “La Sapienza”RomaItaly
  2. 2.Dipartimento di MatematicaUniversità di Roma “Tor Vergata”RomaItaly

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