Abstract
In 1985, Bucher, Ehrenfeucht and Haussler studied derivation relations associated with a given set of context-free rules. Their research motivated a question regarding homomorphisms from the semigroup of all words onto a finite ordered semigroup. The question is which of these homomorphisms induce a well quasi-order on the set of all words. We show that this problem is decidable and the answer does not depend on the homomorphism, but it is a property of the ordered semigroup.
The research was supported by Grant 19-12790S of the Grant Agency of the Czech Republic.
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Notes
- 1.
For a variant of Higman’s Lemma where the alphabet A is equipped with the quasi-order \(\preceq \), we take for the ordered semigroup S the subsemigroup of P(A) consisting of all downward closed subsets of A with respect to the considered quasi-order \(\preceq \). Note that for an infinite alphabet the constructed ordered semigroup is not finite and therefore it is not our focus.
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Acknowledgement
We are grateful to the referees for their numerous valuable suggestions which improved the paper, in particular, its introductory part. We also thank to Michal Kunc for inspiring discussions.
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Klíma, O., Kolegar, J. (2022). Well Quasi-Orders Arising from Finite Ordered Semigroups. In: Diekert, V., Volkov, M. (eds) Developments in Language Theory. DLT 2022. Lecture Notes in Computer Science, vol 13257. Springer, Cham. https://doi.org/10.1007/978-3-031-05578-2_16
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