Issue |
RAIRO-Theor. Inf. Appl.
Volume 40, Number 3, July-September 2006
Word Avoidability Complexity And Morphisms (WACAM)
|
|
---|---|---|
Page(s) | 407 - 426 | |
DOI | https://doi.org/10.1051/ita:2006019 | |
Published online | 18 October 2006 |
Well quasi-orders, unavoidable sets, and derivation systems
1
Dipartimento di Matematica,
Università di Roma “La Sapienza”
Piazzale Aldo Moro 2, 00185 Roma, Italy;
dalessan@mat.uniroma1.it
2
Dipartimento di Matematica,
Università di Roma “Tor Vergata”,
via della Ricerca Scientifica, 00133 Roma, Italy;
varricch@mat.uniroma2.it
Let I be a finite set of words and be the derivation relation
generated by the set of productions {ε → u | u ∈ I}.
Let
be the set of words u such that
.
We prove that the set I is unavoidable if and only if the relation
is a well quasi-order on the set
. This result generalizes a theorem of
[Ehrenfeucht et al.,
Theor. Comput. Sci. 27 (1983) 311–332]. Further generalizations are investigated.
Mathematics Subject Classification: 68Q45 / 68R15
Key words: Well quasi-orders / context-free languages.
© EDP Sciences, 2006
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