Asada and Kobayashi [ICALP 2017] conjectured a higher-order version of Kruskal's tree theorem, and proved a pumping lemma for higher-order languages modulo the conjecture. The conjecture has been proved up to order-2, which implies that Asada and Kobayashi's pumping lemma holds for order-2 tree languages, but remains open for order-3 or higher. In this paper, we prove a variation of the conjecture for order-3. This is sufficient for proving that a variation of the pumping lemma holds for order-3 tree languages (equivalently, for order-4 word languages).
@InProceedings{asada_et_al:LIPIcs.FSTTCS.2018.14, author = {Asada, Kazuyuki and Kobayashi, Naoki}, title = {{Lambda-Definable Order-3 Tree Functions are Well-Quasi-Ordered}}, booktitle = {38th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2018)}, pages = {14:1--14:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-093-4}, ISSN = {1868-8969}, year = {2018}, volume = {122}, editor = {Ganguly, Sumit and Pandya, Paritosh}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2018.14}, URN = {urn:nbn:de:0030-drops-99138}, doi = {10.4230/LIPIcs.FSTTCS.2018.14}, annote = {Keywords: higher-order grammar, pumping lemma, Kruskal's tree theorem, well-quasi-ordering, simply-typed lambda calculus} }
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