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Timed Automata (TA) are a popular formalism to model systems in dense linear time. However, due to their finite state-space, they can only model systems with a finitary logical behavior. There are extensions to e.g., timed pushdown systems and timed recursive state machines. Higher-Order Recursion Schemes (HORS) are another popular model for infinite-state, non-regular systems, naturally stratified by their type-theoretic order. We recently introduced Real-Time Recursion schemes as an approximation of HORS to real-time systems. This paper updates Real-Time Recursion Schemes into Higher-Order Timed Automata, a formalism that defines a tree-shaped timed automaton, which is more suitable to model actual systems. We show that the model-checking problem against the timed version of the modal mu-calculus exhibits the expected complexity bounds, i.e., an increase by one exponential towards the untimed version. We also show that, in the presence of tail recursion, half an exponential can be recovered, mirroring similar gains in the untimed setting. We also give a matching lower bound for the special case of order-1 HORTA. We conjecture that this can be generalized for all orders.
@InProceedings{bruse:LIPIcs.TIME.2025.5,
author = {Bruse, Florian},
title = {{Higher-Order Timed Automata and Tail Recursion}},
booktitle = {32nd International Symposium on Temporal Representation and Reasoning (TIME 2025)},
pages = {5:1--5:16},
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.2025.5},
URN = {urn:nbn:de:0030-drops-244519},
doi = {10.4230/LIPIcs.TIME.2025.5},
annote = {Keywords: Timed Automata, Higher-Order Recursion Schemes, Tree Automata, Tail Recursion}
}