We demonstrate a surprising and first-of-its-kind expressive equivalence between decidable metric and freeze logics over timed words in pointwise semantics. Our main result states that Metric Interval Temporal Logic with future, past and Pnueli modalities, MITPPL, and full unilateral timed propositional temporal logic with both future and past temporal modalities, UPTL, have identical expressiveness. One of the highlights of this paper, which allows for this equivalence, is to prove that UPTL formulas admit monadic decomposition. Our result also implies that several decidable logics for real-time specifications, such as one-variable UPTL, unilateral MITPPL, and Q2MLO, are all expressively equivalent, and the reductions between them are effective. Hence, our result unifies the fragmented expressiveness boundary of timed temporal logics. As corollaries, we resolve the open question of the decidability for full UPTL, and the variable or clock hierarchy problem for the future fragment of UPTL.
@InProceedings{ho_et_al:LIPIcs.CONCUR.2025.24, author = {Ho, Hsi-Ming and Krishna, Shankara Narayanan and Madnani, Khushraj and Majumdar, Rupak and Pandya, Paritosh}, title = {{Expressive Equivalence Between Decidable Freeze and Metric Timed Temporal Logics.}}, booktitle = {36th International Conference on Concurrency Theory (CONCUR 2025)}, pages = {24:1--24:24}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-389-8}, ISSN = {1868-8969}, year = {2025}, volume = {348}, editor = {Bouyer, Patricia and van de Pol, Jaco}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2025.24}, URN = {urn:nbn:de:0030-drops-239744}, doi = {10.4230/LIPIcs.CONCUR.2025.24}, annote = {Keywords: Timed Propositional Temporal Logic, Expressiveness, Metric Interval Temporal Logic, Satisfiability, Decidability} }
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