We introduce strict first-time semantics for the Until operator from linear-time temporal logic which makes assertions not just about some future moment but about the first time in the future that its argument should hold. We investigate Metric Linear-Time Temporal Logic under this interpretation in terms of expressive power, relative succinctness and computational complexity. While the expressiveness does not exceed that of pure LTL, there are properties definable in this logic which can only be expressed in LTL with exponentially larger formulas. Yet, we show that the complexity of the satisfiability problem remains PSPACE-complete which is in contrast to the EXPSPACE-completeness of Metric LTL. The motivation for this logic originates in a study of the expressive power of State Space Models, a recently proposed alternative to the popular transformer architectures in machine learning.
@InProceedings{alsmann_et_al:LIPIcs.TIME.2025.3, author = {Alsmann, Eric and Lange, Martin}, title = {{Metric Linear-Time Temporal Logic with Strict First-Time Semantics}}, booktitle = {32nd International Symposium on Temporal Representation and Reasoning (TIME 2025)}, pages = {3:1--3:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-401-7}, ISSN = {1868-8969}, year = {2025}, volume = {355}, editor = {Vidal, Thierry and Wa{\l}\k{e}ga, Przemys{\l}aw Andrzej}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TIME.2025.3}, URN = {urn:nbn:de:0030-drops-244491}, doi = {10.4230/LIPIcs.TIME.2025.3}, annote = {Keywords: linear-time temporal logic, metric temporal logic, computational complexity, Streett automata} }