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**Published in:** LIPIcs, Volume 119, 27th EACSL Annual Conference on Computer Science Logic (CSL 2018)

In earlier work (LICS 2016), the authors introduced two-variable first-order logic supplemented by a binary relation that allows one to say that a letter appears between two positions. We found an effective algebraic criterion that is a necessary condition for definability in this logic, and conjectured that the criterion is also sufficient, although we proved this only in the case of two-letter alphabets. Here we prove the general conjecture. The proof is quite different from the arguments in the earlier work, and required the development of novel techniques concerning factorizations of words. We extend the results to binary relations specifying that a factor appears between two positions.

Andreas Krebs, Kamal Lodaya, Paritosh K. Pandya, and Howard Straubing. An Algebraic Decision Procedure for Two-Variable Logic with a Between Relation. In 27th EACSL Annual Conference on Computer Science Logic (CSL 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 119, pp. 28:1-28:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{krebs_et_al:LIPIcs.CSL.2018.28, author = {Krebs, Andreas and Lodaya, Kamal and Pandya, Paritosh K. and Straubing, Howard}, title = {{An Algebraic Decision Procedure for Two-Variable Logic with a Between Relation}}, booktitle = {27th EACSL Annual Conference on Computer Science Logic (CSL 2018)}, pages = {28:1--28:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-088-0}, ISSN = {1868-8969}, year = {2018}, volume = {119}, editor = {Ghica, Dan R. and Jung, Achim}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2018.28}, URN = {urn:nbn:de:0030-drops-96953}, doi = {10.4230/LIPIcs.CSL.2018.28}, annote = {Keywords: two-variable logic, finite model theory, algebraic automata theory} }

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**Published in:** LIPIcs, Volume 117, 43rd International Symposium on Mathematical Foundations of Computer Science (MFCS 2018)

We develop team semantics for Linear Temporal Logic (LTL) to express hyperproperties, which have recently been identified as a key concept in the verification of information flow properties. Conceptually, we consider an asynchronous and a synchronous variant of team semantics. We study basic properties of this new logic and classify the computational complexity of its satisfiability, path, and model checking problem. Further, we examine how extensions of these basic logics react on adding other atomic operators. Finally, we compare its expressivity to the one of HyperLTL, another recently introduced logic for hyperproperties. Our results show that LTL under team semantics is a viable alternative to HyperLTL, which complements the expressivity of HyperLTL and has partially better algorithmic properties.

Andreas Krebs, Arne Meier, Jonni Virtema, and Martin Zimmermann. Team Semantics for the Specification and Verification of Hyperproperties. In 43rd International Symposium on Mathematical Foundations of Computer Science (MFCS 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 117, pp. 10:1-10:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{krebs_et_al:LIPIcs.MFCS.2018.10, author = {Krebs, Andreas and Meier, Arne and Virtema, Jonni and Zimmermann, Martin}, title = {{Team Semantics for the Specification and Verification of Hyperproperties}}, booktitle = {43rd International Symposium on Mathematical Foundations of Computer Science (MFCS 2018)}, pages = {10:1--10:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-086-6}, ISSN = {1868-8969}, year = {2018}, volume = {117}, editor = {Potapov, Igor and Spirakis, Paul and Worrell, James}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2018.10}, URN = {urn:nbn:de:0030-drops-95926}, doi = {10.4230/LIPIcs.MFCS.2018.10}, annote = {Keywords: LTL, Hyperproperties, Team Semantics, Model Checking, Satisfiability} }

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**Published in:** LIPIcs, Volume 93, 37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017)

In this work we consider the term evaluation problem which is, given a term over some algebra and a valid input to the term, computing the value of the term on that input. In contrast to previous methods we allow the algebra to be completely general and consider the problem of obtaining an efficient upper bound for this problem. Many variants of the problems where the algebra is well behaved have been studied. For example, the problem over the Boolean semiring or over the semiring (N,+,*). We extend this line of work.
Our efficient term evaluation algorithm then serves as a tool for obtaining polylogarithmic depth upper bounds for various well-studied problems. To demonstrate the utility of our result we show new bounds and reprove known results for a large spectrum of problems. In particular, the applications of the algorithm we consider include (but are not restricted to) arithmetic formula evaluation, word problems for tree and visibly pushdown automata, and various problems related to bounded tree-width and clique-width graphs.

Andreas Krebs, Nutan Limaye, and Michael Ludwig. A Unified Method for Placing Problems in Polylogarithmic Depth. In 37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 93, pp. 36:1-36:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{krebs_et_al:LIPIcs.FSTTCS.2017.36, author = {Krebs, Andreas and Limaye, Nutan and Ludwig, Michael}, title = {{A Unified Method for Placing Problems in Polylogarithmic Depth}}, booktitle = {37th IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2017)}, pages = {36:1--36:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-055-2}, ISSN = {1868-8969}, year = {2018}, volume = {93}, editor = {Lokam, Satya and Ramanujam, R.}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2017.36}, URN = {urn:nbn:de:0030-drops-83805}, doi = {10.4230/LIPIcs.FSTTCS.2017.36}, annote = {Keywords: Polylogarithmic depth, Term evaluation, Parallel algorithms} }

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**Published in:** LIPIcs, Volume 83, 42nd International Symposium on Mathematical Foundations of Computer Science (MFCS 2017)

Cost register automata (CRAs) are one-way finite automata whose transitions have the side effect that a register is set to the result of applying a state-dependent semiring operation to a pair of registers. Here it is shown that CRAs over the tropical semiring (N U {infinity},\min,+) can simulate polynomial time computation, proving along the way that a naturally defined width-k circuit value problem over the tropical semiring is P-complete.
Then the copyless variant of the CRA, requiring that semiring operations be applied to distinct registers, is shown no more powerful than NC^1 when the semiring is (Z,+,x) or (Gamma^*,max,concat). This relates questions left open in recent work on the complexity of CRA-computable functions to long-standing class separation conjectures in complexity theory, such as NC versus P and NC^1 versus GapNC^1.

Eric Allender, Andreas Krebs, and Pierre McKenzie. Better Complexity Bounds for Cost Register Automata. In 42nd International Symposium on Mathematical Foundations of Computer Science (MFCS 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 83, pp. 24:1-24:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{allender_et_al:LIPIcs.MFCS.2017.24, author = {Allender, Eric and Krebs, Andreas and McKenzie, Pierre}, title = {{Better Complexity Bounds for Cost Register Automata}}, booktitle = {42nd International Symposium on Mathematical Foundations of Computer Science (MFCS 2017)}, pages = {24:1--24:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-046-0}, ISSN = {1868-8969}, year = {2017}, volume = {83}, editor = {Larsen, Kim G. and Bodlaender, Hans L. and Raskin, Jean-Francois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2017.24}, URN = {urn:nbn:de:0030-drops-80661}, doi = {10.4230/LIPIcs.MFCS.2017.24}, annote = {Keywords: computational complexity, cost registers} }

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**Published in:** LIPIcs, Volume 87, 25th Annual European Symposium on Algorithms (ESA 2017)

Bounded context switching (BCS) is an under-approximate method for finding violations to safety properties in shared-memory concurrent programs. Technically, BCS is a reachability problem that is known to be NP-complete. Our contribution is a parameterized analysis of BCS.
The first result is an algorithm that solves BCS when parameterized by the number of context switches (cs) and the size of the memory (m) in O*(m^(cs)2^(cs)). This is achieved by creating instances of the easier problem Shuff which we solve via fast subset convolution. We also present a lower bound for BCS of the form m^o(cs / log(cs)), based on the exponential time hypothesis. Interestingly, the gap is closely related to a conjecture that has been open since FOCS'07. Further, we prove that BCS admits no polynomial kernel.
Next, we introduce a measure, called scheduling dimension, that captures the complexity of schedules. We study BCS parameterized by the scheduling dimension (sdim) and show that it can be solved in O*((2m)^(4sdim)4^t), where t is the number of threads. We consider variants of the problem for which we obtain (matching) upper and lower bounds.

Peter Chini, Jonathan Kolberg, Andreas Krebs, Roland Meyer, and Prakash Saivasan. On the Complexity of Bounded Context Switching. In 25th Annual European Symposium on Algorithms (ESA 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 87, pp. 27:1-27:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{chini_et_al:LIPIcs.ESA.2017.27, author = {Chini, Peter and Kolberg, Jonathan and Krebs, Andreas and Meyer, Roland and Saivasan, Prakash}, title = {{On the Complexity of Bounded Context Switching}}, booktitle = {25th Annual European Symposium on Algorithms (ESA 2017)}, pages = {27:1--27:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-049-1}, ISSN = {1868-8969}, year = {2017}, volume = {87}, editor = {Pruhs, Kirk and Sohler, Christian}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2017.27}, URN = {urn:nbn:de:0030-drops-78730}, doi = {10.4230/LIPIcs.ESA.2017.27}, annote = {Keywords: Shared memory concurrency, safety verification, fixed-parameter tractability, exponential time hypothesis, bounded context switching} }

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**Published in:** LIPIcs, Volume 82, 26th EACSL Annual Conference on Computer Science Logic (CSL 2017)

In this paper we relate two generalisations of the finite monoid recognisers of automata theory for the study of circuit complexity classes: Boolean spaces with internal monoids and typed monoids. Using the setting of stamps, this allows us to generalise a number of results from algebraic automata theory as it relates to Büchi's logic on words. We obtain an Eilenberg theorem, a substitution principle based on Stone duality, a block product principle for typed stamps and, as our main result, a topological semidirect product construction, which corresponds to the application of a general form of quantification. These results provide tools for the study of language classes given by logic fragments such as the Boolean circuit complexity classes.

Célia Borlido, Silke Czarnetzki, Mai Gehrke, and Andreas Krebs. Stone Duality and the Substitution Principle. In 26th EACSL Annual Conference on Computer Science Logic (CSL 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 82, pp. 13:1-13:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)

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@InProceedings{borlido_et_al:LIPIcs.CSL.2017.13, author = {Borlido, C\'{e}lia and Czarnetzki, Silke and Gehrke, Mai and Krebs, Andreas}, title = {{Stone Duality and the Substitution Principle}}, booktitle = {26th EACSL Annual Conference on Computer Science Logic (CSL 2017)}, pages = {13:1--13:20}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-045-3}, ISSN = {1868-8969}, year = {2017}, volume = {82}, editor = {Goranko, Valentin and Dam, Mads}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2017.13}, URN = {urn:nbn:de:0030-drops-77060}, doi = {10.4230/LIPIcs.CSL.2017.13}, annote = {Keywords: C-variety of languages, typed monoid, Boolean space with an internal monoid, substitution principle, semidirect product} }

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**Published in:** LIPIcs, Volume 30, 32nd International Symposium on Theoretical Aspects of Computer Science (STACS 2015)

We examine visibly counter languages, which are languages recognized by visibly counter automata (a.k.a. input driven counter automata). We are able to effectively characterize the visibly counter languages in AC^0 and show that they are contained in FO[+].

Andreas Krebs, Klaus-Jörn Lange, and Michael Ludwig. Visibly Counter Languages and Constant Depth Circuits. In 32nd International Symposium on Theoretical Aspects of Computer Science (STACS 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 30, pp. 594-607, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)

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@InProceedings{krebs_et_al:LIPIcs.STACS.2015.594, author = {Krebs, Andreas and Lange, Klaus-J\"{o}rn and Ludwig, Michael}, title = {{Visibly Counter Languages and Constant Depth Circuits}}, booktitle = {32nd International Symposium on Theoretical Aspects of Computer Science (STACS 2015)}, pages = {594--607}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-78-1}, ISSN = {1868-8969}, year = {2015}, volume = {30}, editor = {Mayr, Ernst W. and Ollinger, Nicolas}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2015.594}, URN = {urn:nbn:de:0030-drops-49447}, doi = {10.4230/LIPIcs.STACS.2015.594}, annote = {Keywords: visibly counter automata, constant depth circuits, AC0, FO\lbrack+\rbrack} }

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

We define DLOGTIME proof systems, DLTPS, which generalize NC0 proof systems. It is known that functions such as Exact_k and Majority do not have NC0 proof systems. Here, we give a DLTPS for Exact_k (and therefore for Majority) and also for other natural functions such as Reach and Cliquek. Though many interesting functions have DLTPS, we show that there are languages in NP which do not have DLTPS.
We consider the closure properties of DLTPS and prove that they are closed under union and concatenation but are not closed under intersection and complement. Finally, we consider a hierarchy of polylogarithmic time proof systems and show that the hierarchy is strict.

Andreas Krebs and Nutan Limaye. DLOGTIME Proof Systems. In IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2013). Leibniz International Proceedings in Informatics (LIPIcs), Volume 24, pp. 189-200, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2013)

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@InProceedings{krebs_et_al:LIPIcs.FSTTCS.2013.189, author = {Krebs, Andreas and Limaye, Nutan}, title = {{DLOGTIME Proof Systems}}, booktitle = {IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2013)}, pages = {189--200}, 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.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2013.189}, URN = {urn:nbn:de:0030-drops-43725}, doi = {10.4230/LIPIcs.FSTTCS.2013.189}, annote = {Keywords: Proof systems, DLOGTIME, NC0} }

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**Published in:** LIPIcs, Volume 23, Computer Science Logic 2013 (CSL 2013)

Given two structures G and H distinguishable in FO^k (first-order logic with k variables), let A^k(G,H) denote the minimum alternation depth of a FO^k formula distinguishing G from H. Let A^k(n) be the maximum value of A^k(G,H) over n-element structures. We prove the strictness of the quantifier alternation hierarchy of FO^2 in a strong quantitative form, namely A^2(n) >= n/8-2, which is tight up to a constant factor. For each k >= 2, it holds that A^k(n) > log_(k+1) n-2 even over colored trees, which is also tight up to a constant factor if k >= 3. For k >= 3 the last lower bound holds also over uncolored trees, while the alternation hierarchy of FO^2 collapses even over all uncolored graphs.
We also show examples of colored graphs G and H on n vertices that can be distinguished in FO^2 much more succinctly if the alternation number is increased just by one: while in Sigma_i it is possible to distinguish G from H with bounded quantifier depth, in Pi_i this requires quantifier depth Omega(n2). The quadratic lower bound is best possible here because, if G and H can be distinguished in FO^k with i quantifier alternations, this can be done with quantifier depth n^(2k-2).

Christoph Berkholz, Andreas Krebs, and Oleg Verbitsky. Bounds for the quantifier depth in finite-variable logics: Alternation hierarchy. In Computer Science Logic 2013 (CSL 2013). Leibniz International Proceedings in Informatics (LIPIcs), Volume 23, pp. 61-80, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2013)

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@InProceedings{berkholz_et_al:LIPIcs.CSL.2013.61, author = {Berkholz, Christoph and Krebs, Andreas and Verbitsky, Oleg}, title = {{Bounds for the quantifier depth in finite-variable logics: Alternation hierarchy}}, booktitle = {Computer Science Logic 2013 (CSL 2013)}, pages = {61--80}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-60-6}, ISSN = {1868-8969}, year = {2013}, volume = {23}, editor = {Ronchi Della Rocca, Simona}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2013.61}, URN = {urn:nbn:de:0030-drops-41907}, doi = {10.4230/LIPIcs.CSL.2013.61}, annote = {Keywords: Alternation hierarchy, finite-variable logic} }

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**Published in:** LIPIcs, Volume 18, IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2012)

We characterize the languages in the individual levels of the quantifier alternation hierarchy of first-order logic with two variables by identities. This implies decidability of the individual levels. More generally we show that two-sided semidirect products with J as the right-hand factor preserve decidability.

Andreas Krebs and Howard Straubing. An effective characterization of the alternation hierarchy in two-variable logic. In IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2012). Leibniz International Proceedings in Informatics (LIPIcs), Volume 18, pp. 86-98, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2012)

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@InProceedings{krebs_et_al:LIPIcs.FSTTCS.2012.86, author = {Krebs, Andreas and Straubing, Howard}, title = {{An effective characterization of the alternation hierarchy in two-variable logic}}, booktitle = {IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science (FSTTCS 2012)}, pages = {86--98}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-47-7}, ISSN = {1868-8969}, year = {2012}, volume = {18}, editor = {D'Souza, Deepak and Radhakrishnan, Jaikumar and Telikepalli, Kavitha}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2012.86}, URN = {urn:nbn:de:0030-drops-38501}, doi = {10.4230/LIPIcs.FSTTCS.2012.86}, annote = {Keywords: FO\underline2, Quantifier Alternation, J, Pseudovarities, Identities} }