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Invited Paper

**Published in:** LIPIcs, Volume 279, 34th International Conference on Concurrency Theory (CONCUR 2023)

This short article recaps the purpose of the CONCUR Test-of-Time Award and presents the paper that received the Award in 2023.

Bengt Jonsson, Marta Kwiatkowska, and Igor Walukiewicz. CONCUR Test-Of-Time Award 2023 (Invited Paper). In 34th International Conference on Concurrency Theory (CONCUR 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 279, pp. 1:1-1:2, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{jonsson_et_al:LIPIcs.CONCUR.2023.1, author = {Jonsson, Bengt and Kwiatkowska, Marta and Walukiewicz, Igor}, title = {{CONCUR Test-Of-Time Award 2023}}, booktitle = {34th International Conference on Concurrency Theory (CONCUR 2023)}, pages = {1:1--1:2}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-299-0}, ISSN = {1868-8969}, year = {2023}, volume = {279}, editor = {P\'{e}rez, Guillermo A. and Raskin, Jean-Fran\c{c}ois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2023.1}, URN = {urn:nbn:de:0030-drops-189953}, doi = {10.4230/LIPIcs.CONCUR.2023.1}, annote = {Keywords: CONCUR Test-of-Time Award} }

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**Published in:** LIPIcs, Volume 279, 34th International Conference on Concurrency Theory (CONCUR 2023)

In parametric lock-sharing systems processes can spawn new processes to run in parallel, and can create new locks. The behavior of every process is given by a pushdown automaton. We consider infinite behaviors of such systems under strong process fairness condition. A result of a potentially infinite execution of a system is a limit configuration, that is a potentially infinite tree. The verification problem is to determine if a given system has a limit configuration satisfying a given regular property. This formulation of the problem encompasses verification of reachability as well as of many liveness properties. We show that this verification problem, while undecidable in general, is decidable for nested lock usage.
We show Exptime-completeness of the verification problem. The main source of complexity is the number of parameters in the spawn operation. If the number of parameters is bounded, our algorithm works in Ptime for properties expressed by parity automata with a fixed number of ranks.

Corto Mascle, Anca Muscholl, and Igor Walukiewicz. Model-Checking Parametric Lock-Sharing Systems Against Regular Constraints. In 34th International Conference on Concurrency Theory (CONCUR 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 279, pp. 24:1-24:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{mascle_et_al:LIPIcs.CONCUR.2023.24, author = {Mascle, Corto and Muscholl, Anca and Walukiewicz, Igor}, title = {{Model-Checking Parametric Lock-Sharing Systems Against Regular Constraints}}, booktitle = {34th International Conference on Concurrency Theory (CONCUR 2023)}, pages = {24:1--24:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-299-0}, ISSN = {1868-8969}, year = {2023}, volume = {279}, editor = {P\'{e}rez, Guillermo A. and Raskin, Jean-Fran\c{c}ois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2023.24}, URN = {urn:nbn:de:0030-drops-190184}, doi = {10.4230/LIPIcs.CONCUR.2023.24}, annote = {Keywords: Parametric systems, Locks, Model-checking} }

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**Published in:** LIPIcs, Volume 243, 33rd International Conference on Concurrency Theory (CONCUR 2022)

We study the Büchi non-emptiness problem for networks of timed automata. Standard solutions consider the network as a monolithic timed automaton obtained as a synchronized product and build its zone graph on-the-fly under the classical global-time semantics. In the global-time semantics, all processes are assumed to have a common global timeline.
Bengtsson et al. in 1998 have proposed a local-time semantics where each process in the network moves independently according to a local timeline, and processes synchronize their timelines when they do a common action. It has been shown that the local-time semantics is equivalent to the global-time semantics for finite runs, and hence can be used for checking reachability. The local-time semantics allows computation of a local zone graph which has good independence properties and is amenable to partial-order methods. Hence local zone graphs are able to better tackle the state-space explosion due to concurrency.
In this work, we extend the results to the Büchi setting. We propose a local zone graph computation that can be coupled with a partial-order method, to solve the Büchi non-emptiness problem in timed networks. In the process, we develop a theory of regions for the local-time semantics.

Frédéric Herbreteau, B. Srivathsan, and Igor Walukiewicz. Checking Timed Büchi Automata Emptiness Using the Local-Time Semantics. In 33rd International Conference on Concurrency Theory (CONCUR 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 243, pp. 12:1-12:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{herbreteau_et_al:LIPIcs.CONCUR.2022.12, author = {Herbreteau, Fr\'{e}d\'{e}ric and Srivathsan, B. and Walukiewicz, Igor}, title = {{Checking Timed B\"{u}chi Automata Emptiness Using the Local-Time Semantics}}, booktitle = {33rd International Conference on Concurrency Theory (CONCUR 2022)}, pages = {12:1--12:24}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-246-4}, ISSN = {1868-8969}, year = {2022}, volume = {243}, editor = {Klin, Bartek and Lasota, S{\l}awomir and Muscholl, Anca}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2022.12}, URN = {urn:nbn:de:0030-drops-170756}, doi = {10.4230/LIPIcs.CONCUR.2022.12}, annote = {Keywords: Timed B\"{u}chi automata, local-time semantics, zones, abstraction, partial-order reduction} }

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Track B: Automata, Logic, Semantics, and Theory of Programming

**Published in:** LIPIcs, Volume 229, 49th International Colloquium on Automata, Languages, and Programming (ICALP 2022)

We consider the distributed control synthesis problem for systems with locks. The goal is to find local controllers so that the global system does not deadlock. With no restriction this problem is undecidable even for three processes each using a fixed number of locks. We propose two restrictions that make distributed control decidable. The first one is to allow each process to use at most two locks. The problem then becomes complete for the second level of the polynomial time hierarchy, and even in Ptime under some additional assumptions. The dining philosophers problem satisfies these assumptions. The second restriction is a nested usage of locks. In this case the synthesis problem is Nexptime-complete. The drinking philosophers problem falls in this case.

Hugo Gimbert, Corto Mascle, Anca Muscholl, and Igor Walukiewicz. Distributed Controller Synthesis for Deadlock Avoidance. In 49th International Colloquium on Automata, Languages, and Programming (ICALP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 229, pp. 125:1-125:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{gimbert_et_al:LIPIcs.ICALP.2022.125, author = {Gimbert, Hugo and Mascle, Corto and Muscholl, Anca and Walukiewicz, Igor}, title = {{Distributed Controller Synthesis for Deadlock Avoidance}}, booktitle = {49th International Colloquium on Automata, Languages, and Programming (ICALP 2022)}, pages = {125:1--125:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-235-8}, ISSN = {1868-8969}, year = {2022}, volume = {229}, editor = {Boja\'{n}czyk, Miko{\l}aj and Merelli, Emanuela and Woodruff, David P.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2022.125}, URN = {urn:nbn:de:0030-drops-164668}, doi = {10.4230/LIPIcs.ICALP.2022.125}, annote = {Keywords: Distributed Synthesis, Concurrency, Lock Synchronisation, Deadlock Avoidance} }

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**Published in:** LIPIcs, Volume 171, 31st International Conference on Concurrency Theory (CONCUR 2020)

The Heard-Of model is a simple and relatively expressive model of distributed computation. Because of this, it has gained a considerable attention of the verification community. We give a characterization of all algorithms solving consensus in a fragment of this model. The fragment is big enough to cover many prominent consensus algorithms. The characterization is purely syntactic: it is expressed in terms of some conditions on the text of the algorithm.

A. R. Balasubramanian and Igor Walukiewicz. Characterizing Consensus in the Heard-Of Model. In 31st International Conference on Concurrency Theory (CONCUR 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 171, pp. 9:1-9:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)

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@InProceedings{balasubramanian_et_al:LIPIcs.CONCUR.2020.9, author = {Balasubramanian, A. R. and Walukiewicz, Igor}, title = {{Characterizing Consensus in the Heard-Of Model}}, booktitle = {31st International Conference on Concurrency Theory (CONCUR 2020)}, pages = {9:1--9:18}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-160-3}, ISSN = {1868-8969}, year = {2020}, volume = {171}, editor = {Konnov, Igor and Kov\'{a}cs, Laura}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2020.9}, URN = {urn:nbn:de:0030-drops-128217}, doi = {10.4230/LIPIcs.CONCUR.2020.9}, annote = {Keywords: consensus problem, Heard-Of model, verification} }

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**Published in:** LIPIcs, Volume 140, 30th International Conference on Concurrency Theory (CONCUR 2019)

We investigate a zone based approach for the reachability problem in timed automata. The challenge is to alleviate the size explosion of the search space when considering networks of timed automata working in parallel. In the timed setting this explosion is particularly visible as even different interleavings of local actions of processes may lead to different zones. Salah et al. in 2006 have shown that the union of all these different zones is also a zone. This observation was used in an algorithm which from time to time detects and aggregates these zones into a single zone.
We show that such aggregated zones can be calculated more efficiently using the local time semantics and the related notion of local zones proposed by Bengtsson et al. in 1998. Next, we point out a flaw in the existing method to ensure termination of the local zone graph computation. We fix this with a new algorithm that builds the local zone graph and uses abstraction techniques over (standard) zones for termination. We evaluate our algorithm on standard examples. On various examples, we observe an order of magnitude decrease in the search space. On the other examples, the algorithm performs like the standard zone algorithm.

R. Govind, Frédéric Herbreteau, B. Srivathsan, and Igor Walukiewicz. Revisiting Local Time Semantics for Networks of Timed Automata. In 30th International Conference on Concurrency Theory (CONCUR 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 140, pp. 16:1-16:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{govind_et_al:LIPIcs.CONCUR.2019.16, author = {Govind, R. and Herbreteau, Fr\'{e}d\'{e}ric and Srivathsan, B. and Walukiewicz, Igor}, title = {{Revisiting Local Time Semantics for Networks of Timed Automata}}, booktitle = {30th International Conference on Concurrency Theory (CONCUR 2019)}, pages = {16:1--16:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-121-4}, ISSN = {1868-8969}, year = {2019}, volume = {140}, editor = {Fokkink, Wan and van Glabbeek, Rob}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2019.16}, URN = {urn:nbn:de:0030-drops-109184}, doi = {10.4230/LIPIcs.CONCUR.2019.16}, annote = {Keywords: Timed automata, verification, local-time semantics, abstraction} }

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**Published in:** LIPIcs, Volume 65, 36th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2016)

The liveness problem for timed automata asks if a given automaton has a run passing infinitely often through an accepting state. We show that unless P=NP, the liveness problem is more difficult than the reachability problem; more precisely, we exhibit a family of automata for which solving the reachability problem with the standard algorithm is in P but solving the liveness problem is NP-hard. This leads us to revisit the algorithmics for the liveness problem. We propose a notion of a witness for the fact that a timed automaton violates a liveness property. We give an algorithm for computing such a witness and compare it with the existing solutions.

Frédéric Herbreteau, B. Srivathsan, Thanh-Tung Tran, and Igor Walukiewicz. Why Liveness for Timed Automata Is Hard, and What We Can Do About It. In 36th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 65, pp. 48:1-48:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{herbreteau_et_al:LIPIcs.FSTTCS.2016.48, author = {Herbreteau, Fr\'{e}d\'{e}ric and Srivathsan, B. and Tran, Thanh-Tung and Walukiewicz, Igor}, title = {{Why Liveness for Timed Automata Is Hard, and What We Can Do About It}}, booktitle = {36th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2016)}, pages = {48:1--48:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-027-9}, ISSN = {1868-8969}, year = {2016}, volume = {65}, editor = {Lal, Akash and Akshay, S. and Saurabh, Saket and Sen, Sandeep}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2016.48}, URN = {urn:nbn:de:0030-drops-68831}, doi = {10.4230/LIPIcs.FSTTCS.2016.48}, annote = {Keywords: Timed automata, model-checking, liveness invariant, state subsumption} }

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**Published in:** LIPIcs, Volume 59, 27th International Conference on Concurrency Theory (CONCUR 2016)

Negotiations are a formalism for describing multiparty distributed cooperation. Alternatively, they can be seen as a model of concurrency with synchronized choice as communication primitive. Well-designed negotiations must be sound, meaning that, whatever its current state, the negotiation can still be completed. In a former paper, Esparza and Desel have shown that deciding soundness of a negotiation is PSPACE-complete, and in PTIME if the negotiation is deterministic. They have also provided an algorithm for an intermediate class of acyclic, non-deterministic negotiations, but left the complexity of the soundness problem open.
In the first part of this paper we study two further analysis problems for sound acyclic deterministic negotiations, called the race and the omission problem, and give polynomial algorithms. We use these results to provide the first polynomial algorithm for some analysis problems of workflow nets with data previously studied by Trcka, van der Aalst, and Sidorova.
In the second part we solve the open question of Esparza and Desel's paper. We show that soundness of acyclic, weakly non-deterministic negotiations is in PTIME, and that checking soundness is already NP-complete for slightly more general classes.

Javier Esparza, Denis Kuperberg, Anca Muscholl, and Igor Walukiewicz. Soundness in Negotiations. In 27th International Conference on Concurrency Theory (CONCUR 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 59, pp. 12:1-12:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{esparza_et_al:LIPIcs.CONCUR.2016.12, author = {Esparza, Javier and Kuperberg, Denis and Muscholl, Anca and Walukiewicz, Igor}, title = {{Soundness in Negotiations}}, booktitle = {27th International Conference on Concurrency Theory (CONCUR 2016)}, pages = {12:1--12:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-017-0}, ISSN = {1868-8969}, year = {2016}, volume = {59}, editor = {Desharnais, Jos\'{e}e and Jagadeesan, Radha}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2016.12}, URN = {urn:nbn:de:0030-drops-61636}, doi = {10.4230/LIPIcs.CONCUR.2016.12}, annote = {Keywords: Negotiations, workflows, soundness, verification, concurrency} }

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**Published in:** LIPIcs, Volume 55, 43rd International Colloquium on Automata, Languages, and Programming (ICALP 2016)

We study the topological complexity of languages of Büchi automata on infinite binary trees. We show that such a language is either Borel and WMSO-definable, or Sigma_1^1-complete and not WMSO-definable; moreover it can be algorithmically decided which of the two cases holds. The proof relies on a direct reduction to deciding the winner in a finite game with a regular winning condition.

Michal Skrzypczak and Igor Walukiewicz. Deciding the Topological Complexity of Büchi Languages. In 43rd International Colloquium on Automata, Languages, and Programming (ICALP 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 55, pp. 99:1-99:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2016)

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@InProceedings{skrzypczak_et_al:LIPIcs.ICALP.2016.99, author = {Skrzypczak, Michal and Walukiewicz, Igor}, title = {{Deciding the Topological Complexity of B\"{u}chi Languages}}, booktitle = {43rd International Colloquium on Automata, Languages, and Programming (ICALP 2016)}, pages = {99:1--99:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-013-2}, ISSN = {1868-8969}, year = {2016}, volume = {55}, editor = {Chatzigiannakis, Ioannis and Mitzenmacher, Michael and Rabani, Yuval and Sangiorgi, Davide}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2016.99}, URN = {urn:nbn:de:0030-drops-62346}, doi = {10.4230/LIPIcs.ICALP.2016.99}, annote = {Keywords: tree automata, non-determinism, Borel sets, topological complexity, decidability} }

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**Published in:** LIPIcs, Volume 45, 35th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2015)

We define a new class of pushdown systems where the pushdown is a tree instead of a word. We allow a limited form of lookahead on the pushdown conforming to a certain ordering restriction, and we show that the resulting class enjoys a decidable reachability problem. This follows from a preservation of recognizability result for the backward reachability relation of such systems. As an application, we show that our simple model can encode several formalisms generalizing pushdown systems, such as ordered multi-pushdown systems, annotated higher-order pushdown systems, the Krivine machine, and ordered annotated multi-pushdown systems. In each case, our procedure yields tight complexity.

Lorenzo Clemente, Pawel Parys, Sylvain Salvati, and Igor Walukiewicz. Ordered Tree-Pushdown Systems. In 35th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 45, pp. 163-177, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{clemente_et_al:LIPIcs.FSTTCS.2015.163, author = {Clemente, Lorenzo and Parys, Pawel and Salvati, Sylvain and Walukiewicz, Igor}, title = {{Ordered Tree-Pushdown Systems}}, booktitle = {35th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2015)}, pages = {163--177}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-97-2}, ISSN = {1868-8969}, year = {2015}, volume = {45}, editor = {Harsha, Prahladh and Ramalingam, G.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2015.163}, URN = {urn:nbn:de:0030-drops-56470}, doi = {10.4230/LIPIcs.FSTTCS.2015.163}, annote = {Keywords: reachability analysis, saturation technique, pushdown automata, ordered pushdown automata, higher-order pushdown automata, higher-order recursive sche} }

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**Published in:** LIPIcs, Volume 41, 24th EACSL Annual Conference on Computer Science Logic (CSL 2015)

We consider simply typed lambda-calculus with fixpoints as a non-interpreted functional programming language: the result of the execution of a program is its normal form that can be seen as a potentially infinite tree of calls to built-in operations. Properties of such trees are properties of executions of programs and monadic second-order logic (MSOL) is well suited to express them.
For a given MSOL property we show how to construct a finitary model recognizing it. In other words, the value of a lambda-term in the model determines if the tree that is the result of the execution of the term satisfies the property. The finiteness of the construction has as consequences many known results about the verification of higher-order programs in this framework.

Sylvain Salvati and Igor Walukiewicz. A Model for Behavioural Properties of Higher-order Programs. In 24th EACSL Annual Conference on Computer Science Logic (CSL 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 41, pp. 229-243, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{salvati_et_al:LIPIcs.CSL.2015.229, author = {Salvati, Sylvain and Walukiewicz, Igor}, title = {{A Model for Behavioural Properties of Higher-order Programs}}, booktitle = {24th EACSL Annual Conference on Computer Science Logic (CSL 2015)}, pages = {229--243}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-90-3}, ISSN = {1868-8969}, year = {2015}, volume = {41}, editor = {Kreutzer, Stephan}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2015.229}, URN = {urn:nbn:de:0030-drops-54173}, doi = {10.4230/LIPIcs.CSL.2015.229}, annote = {Keywords: Simply typed lambda-Y-calculus, Monadic second order logic, semantic models} }

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**Published in:** LIPIcs, Volume 42, 26th International Conference on Concurrency Theory (CONCUR 2015)

Verification of concurrent systems is a difficult problem in general, and this is the case even more in a parametrized setting where unboundedly many concurrent components are considered. Recently, Hague proposed an architecture with a leader process and unboundedly many copies of a contributor process interacting over a shared memory for which safety properties can be effectively verified. All processes in Hague's setting are pushdown automata. Here, we extend it by considering other formal models and, as a main contribution, find very liberal conditions on the individual processes under which the safety problem is decidable: the only substantial condition we require is the effective computability of the downward closure for the class of the leader processes. Furthermore, our result allows for a hierarchical approach to constructing models of concurrent systems with decidable safety problem: networks with tree-like architecture, where each process shares a register with its children processes (and another register with its parent). Nodes in such networks can be for instance pushdown automata, Petri nets, or multi-pushdown systems with decidable reachability problem.

Salvatore La Torre, Anca Muscholl, and Igor Walukiewicz. Safety of Parametrized Asynchronous Shared-Memory Systems is Almost Always Decidable. In 26th International Conference on Concurrency Theory (CONCUR 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 42, pp. 72-84, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{latorre_et_al:LIPIcs.CONCUR.2015.72, author = {La Torre, Salvatore and Muscholl, Anca and Walukiewicz, Igor}, title = {{Safety of Parametrized Asynchronous Shared-Memory Systems is Almost Always Decidable}}, booktitle = {26th International Conference on Concurrency Theory (CONCUR 2015)}, pages = {72--84}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-91-0}, ISSN = {1868-8969}, year = {2015}, volume = {42}, editor = {Aceto, Luca and de Frutos Escrig, David}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.CONCUR.2015.72}, URN = {urn:nbn:de:0030-drops-53813}, doi = {10.4230/LIPIcs.CONCUR.2015.72}, annote = {Keywords: Verification, parametrized systems, shared memory} }

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**Published in:** LIPIcs, Volume 29, 34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014)

The distributed synthesis problem is about constructing correct distributed systems, i.e., systems that satisfy a given specification. We consider a slightly more general problem of distributed control, where the goal is to restrict the behavior of a given distributed system in order to satisfy the specification. Our systems are finite state machines that communicate via rendez-vous (Zielonka automata). We show decidability of the synthesis problem for all omega-regular local specifications, under the restriction that the communication graph of the system is acyclic. This result extends a previous decidability result for a restricted form of local reachability specifications.

Anca Muscholl and Igor Walukiewicz. Distributed Synthesis for Acyclic Architectures. In 34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014). Leibniz International Proceedings in Informatics (LIPIcs), Volume 29, pp. 639-651, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2014)

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@InProceedings{muscholl_et_al:LIPIcs.FSTTCS.2014.639, author = {Muscholl, Anca and Walukiewicz, Igor}, title = {{Distributed Synthesis for Acyclic Architectures}}, booktitle = {34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014)}, pages = {639--651}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-77-4}, ISSN = {1868-8969}, year = {2014}, volume = {29}, editor = {Raman, Venkatesh and Suresh, S. P.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2014.639}, URN = {urn:nbn:de:0030-drops-48772}, doi = {10.4230/LIPIcs.FSTTCS.2014.639}, annote = {Keywords: Distributed synthesis, distributed control, causal memory} }

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**Published in:** LIPIcs, Volume 24, IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2013)

We consider simply-typed lambda calculus with fixpoint operators.
Evaluation of a term gives as a result the Böhm tree of the term.
We show that evaluation is compatible with monadic second-order logic (MSOL). This means that for a fixed finite vocabulary of terms, the MSOL properties of Böhm trees of terms are effectively MSOL properties of terms themselves. Theorems of this kind have been known for some graph operations: unfolding, and Muchnik iteration. Similarly to those results, our main theorem has diverse applications. It can be used to show decidability results, to construct classes of graphs with decidable MSOL theory, or to obtain MSOL formulas expressing behavioral properties of terms. Another application is decidability of a control-flow synthesis problem.

Sylvain Salvati and Igor Walukiewicz. Evaluation is MSOL-compatible. In IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2013). Leibniz International Proceedings in Informatics (LIPIcs), Volume 24, pp. 103-114, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2013)

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@InProceedings{salvati_et_al:LIPIcs.FSTTCS.2013.103, author = {Salvati, Sylvain and Walukiewicz, Igor}, title = {{Evaluation is MSOL-compatible}}, booktitle = {IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2013)}, pages = {103--114}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-64-4}, ISSN = {1868-8969}, year = {2013}, volume = {24}, editor = {Seth, Anil and Vishnoi, Nisheeth K.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2013.103}, URN = {urn:nbn:de:0030-drops-43652}, doi = {10.4230/LIPIcs.FSTTCS.2013.103}, annote = {Keywords: Simply typed \$lambda Y\$-calculus; Monadic second order} }

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**Published in:** LIPIcs, Volume 13, IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2011)

The reachability problem for timed automata asks if there exists a
path from an initial state to a target state. The standard
solution to this problem involves computing the zone graph of the
automaton, which in principle could be infinite. In order to make
the graph finite, zones are approximated using an extrapolation
operator. For reasons of efficiency in current algorithms
extrapolation of a zone is always a zone; and in particular it
is convex.
In this paper, we propose to solve the reachability problem without
such extrapolation operators. To ensure termination, we provide an
efficient algorithm to check if a zone is included in the so
called region closure of another. Although theoretically better,
closure cannot be used in the standard algorithm since a closure of
a zone may not be convex.
An additional benefit of the proposed approach is that it permits to
calculate approximating parameters on-the-fly during exploration of
the zone graph, as opposed to the current methods which do it by a
static analysis of the automaton prior to the exploration. This
allows for further improvements in the algorithm. Promising
experimental results are presented.

Frédéric Herbreteau, Dileep Kini, B. Srivathsan, and Igor Walukiewicz. Using non-convex approximations for efficient analysis of timed automata. In IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2011). Leibniz International Proceedings in Informatics (LIPIcs), Volume 13, pp. 78-89, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2011)

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@InProceedings{herbreteau_et_al:LIPIcs.FSTTCS.2011.78, author = {Herbreteau, Fr\'{e}d\'{e}ric and Kini, Dileep and Srivathsan, B. and Walukiewicz, Igor}, title = {{Using non-convex approximations for efficient analysis of timed automata}}, booktitle = {IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2011)}, pages = {78--89}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-34-7}, ISSN = {1868-8969}, year = {2011}, volume = {13}, editor = {Chakraborty, Supratik and Kumar, Amit}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2011.78}, URN = {urn:nbn:de:0030-drops-33619}, doi = {10.4230/LIPIcs.FSTTCS.2011.78}, annote = {Keywords: Timed Automata, Model-checking, Non-convex abstraction, On-the-fly abstraction} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 10252, Game Semantics and Program Verification (2010)

From 20th to 25th June 2010, the Dagstuhl Seminar
"Game Semantics and Program Verification'' was held
in Schloss Dagstuhl - Leibniz Center for Informatics.
During the seminar, several participants presented their current
research, and ongoing work and open problems were discussed.
Abstracts of the presentations given during the seminar
as well as abstracts of seminar results and ideas are put
together in this paper. The first section
describes the seminar topics and goals in general.
Links to extended abstracts or full papers are provided, if available.

Paul-Andre Mellies, Andrzej S. Murawski, Andrea Schalk, and Igor Walukiewicz. 10252 Abstracts Collection – Game Semantics and Program Verification. In Game Semantics and Program Verification. Dagstuhl Seminar Proceedings, Volume 10252, pp. 1-11, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{mellies_et_al:DagSemProc.10252.1, author = {Mellies, Paul-Andre and Murawski, Andrzej S. and Schalk, Andrea and Walukiewicz, Igor}, title = {{10252 Abstracts Collection – Game Semantics and Program Verification}}, booktitle = {Game Semantics and Program Verification}, pages = {1--11}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2010}, volume = {10252}, editor = {Paul-Andre Mellies and Andrzej S. Murawski and Andrea Schalk and Igor Walukiewicz}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/DagSemProc.10252.1}, URN = {urn:nbn:de:0030-drops-27945}, doi = {10.4230/DagSemProc.10252.1}, annote = {Keywords: Software verification, Semantics of programming languages, Game semantics, Static analysis, Model checking} }

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**Published in:** Dagstuhl Seminar Proceedings, Volume 10252, Game Semantics and Program Verification (2010)

The seminar took place from 20th until 25th June 2010.
Its primary aim was to foster interaction between researchers
working on modelling programs/proofs using games and
the verification community. The meeting brought together
28 researchers from eight different countries,
both junior and senior, for a systematic assessment of what
the two areas have to offer to one another,
critical evaluation of what has been achieved so far,
with a view to establishing common research goals for the future.

Paul-Andre Mellies, Andrzej S. Murawski, Andrea Schalk, and Igor Walukiewicz. 10252 Executive Summary – Game Semantics and Program Verification. In Game Semantics and Program Verification. Dagstuhl Seminar Proceedings, Volume 10252, pp. 1-5, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2010)

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@InProceedings{mellies_et_al:DagSemProc.10252.2, author = {Mellies, Paul-Andre and Murawski, Andrzej S. and Schalk, Andrea and Walukiewicz, Igor}, title = {{10252 Executive Summary – Game Semantics and Program Verification}}, booktitle = {Game Semantics and Program Verification}, pages = {1--5}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2010}, volume = {10252}, editor = {Paul-Andre Mellies and Andrzej S. Murawski and Andrea Schalk and Igor Walukiewicz}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops-dev.dagstuhl.de/entities/document/10.4230/DagSemProc.10252.2}, URN = {urn:nbn:de:0030-drops-27938}, doi = {10.4230/DagSemProc.10252.2}, annote = {Keywords: Software verification, Semantics of programming languages, Game semantics, Static analysis, Model checking} }

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