no. 3
Well quasi-orders, unavoidable sets, and derivation systems
RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 40 (2006) no. 3, pp. 407-426.

Let I be a finite set of words and ⇒ I * be the derivation relation generated by the set of productions {ϵ→u∣u∈I}. Let L I ϵ be the set of words u such that ϵ⇒ I * u. We prove that the set I is unavoidable if and only if the relation ⇒ I * is a well quasi-order on the set L I ϵ . This result generalizes a theorem of [Ehrenfeucht et al., Theor. Comput. Sci. 27 (1983) 311-332]. Further generalizations are investigated.

DOI : 10.1051/ita:2006019
Classification : 68Q45, 68R15
Mots clés : well quasi-orders, context-free languages 
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D'Alessandro, Flavio; Varricchio, Stefano. Well quasi-orders, unavoidable sets, and derivation systems. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 40 (2006) no. 3, pp. 407-426. doi : 10.1051/ita:2006019. http://archive.numdam.org/articles/10.1051/ita:2006019/

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