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We show that every confluent abstract rewriting system (ARS) of the cardinality that does not exceed the first uncountable cardinal belongs to the class DCR₃, i.e. the class of confluent ARS for which confluence can be proved with the the help of the decreasing diagrams method using the set of labels {0,1,2} ordered in such a way that 0<1<2 (in the general case, the decreasing diagrams method with two labels is not sufficient for proving confluence of such ARS). Under the Continuum Hypothesis this result implies that the decreasing diagrams method is sufficient for establishing confluence of ARS on many structures of interest to applied mathematics and various interdisciplinary fields (confluence of ARS on real numbers, continuous real functions, etc.).
We provide a machine-checked formal proof of a formalized version of the main result in Isabelle proof assistant using HOL logic and the HOL-Cardinals theory. An extended version of this formalization is available in the Archive of Formal Proofs.
@InProceedings{ivanov:LIPIcs.FSCD.2025.25,
author = {Ivanov, Ievgen},
title = {{Completeness of the Decreasing Diagrams Method for Proving Confluence of Rewriting Systems of the Least Uncountable Cardinality}},
booktitle = {10th International Conference on Formal Structures for Computation and Deduction (FSCD 2025)},
pages = {25:1--25:20},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-374-4},
ISSN = {1868-8969},
year = {2025},
volume = {337},
editor = {Fern\'{a}ndez, Maribel},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2025.25},
URN = {urn:nbn:de:0030-drops-236404},
doi = {10.4230/LIPIcs.FSCD.2025.25},
annote = {Keywords: confluence, decreasing diagrams method, rewriting systems, reduction, formal methods, formal proofs, formal verification, non-discrete models, nondeterministic models, interval models}
}