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Brief Announcement: Time-Travel Planning with Tenet Turnstiles

Authors Thibaut Blanc , Quentin Bramas , Jean-Romain Luttringer , Sébastien Tixeuil

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Subject Classification

ACM Subject Classification
  • Theory of computation → Dynamic graph algorithms
Keywords
  • dynamic graphs
  • time travel
  • online algorithms

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Abstract

We study routing in dynamic graphs when an agent may use backward time travel (BTT) devices. Minimizing delay (arrival time minus departure time) is the primary objective; the number of time inversions is the secondary cost. Building on the framework of Bramas et al., we introduce two space-time online settings - ST-online-easy and ST-online-hard - and analyze the Tenet model, where BTT is performed by entering a turnstile that reverses the direction of time flow. We obtain a polynomial-time offline algorithm, tight competitive ratios for the T-online and S-online settings, a tight quadratic competitive ratio for ST-online-easy, and we prove that no finite competitive ratio exists for ST-online-hard.

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Thibaut Blanc, Quentin Bramas, Jean-Romain Luttringer, and Sébastien Tixeuil. Brief Announcement: Time-Travel Planning with Tenet Turnstiles. In 5th Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 373, pp. 22:1-22:6, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026) https://doi.org/10.4230/LIPIcs.SAND.2026.22

Author Details

Thibaut Blanc
  • ENS Lyon, France
Quentin Bramas
  • Strasbourg University, CNRS, ICUBE, France
Jean-Romain Luttringer
  • Strasbourg University, CNRS, ICUBE, France
Sébastien Tixeuil
  • Sorbonne University, CNRS, LIP6, IUF, France

References

  1. Quentin Bramas, Jean-Romain Luttringer, and Sébastien Tixeuil. Offline constrained backward time travel planning. In Shlomi Dolev and Baruch Schieber, editors, Stabilization, Safety, and Security of Distributed Systems - 25th International Symposium, SSS 2023, Jersey City, NJ, USA, October 2-4, 2023, Proceedings, volume 14310 of Lecture Notes in Computer Science, pages 466-480. Springer, 2023. URL: https://doi.org/10.1007/978-3-031-44274-2_35.
  2. Quentin Bramas, Jean-Romain Luttringer, and Sébastien Tixeuil. Online space-time travel planning in dynamic graphs. In Arnaud Casteigts and Fabian Kuhn, editors, 3rd Symposium on Algorithmic Foundations of Dynamic Networks (SAND 2024), volume 292 of Leibniz International Proceedings in Informatics (LIPIcs), pages 7:1-7:14. Schloss Dagstuhl - Leibniz-Zentrum für Informatik, 2024. URL: https://doi.org/10.4230/LIPIcs.SAND.2024.7.
  3. Quentin Bramas, Jean-Romain Luttringer, and Sébastien Tixeuil. On time-travel planning in dynamic graphs. Theor. Comput. Sci., 1054:115501, 2025. URL: https://doi.org/10.1016/J.TCS.2025.115501.
  4. Christopher Nolan. Tenet, 2020. Google Scholar
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