# Conference Publications

## 2020

- Patch Graph RewritingRoy Overbeek, and Jörg EndrullisIn: Proc. Conf. on Graph Transformation (ICGT 2020), pp. 128–145, Springer (2020)paper
# Summary

The basic principle of graph rewriting is the stepwise replacement of subgraphs inside a host graph. A challenge in such replacement steps is the treatment of the patch graph, consisting of those edges of the host graph that touch the subgraph, but are not part of it.

We introduce patch graph rewriting, a visual graph rewriting language with precise formal semantics. The language has rich expressive power in two ways. First, rewrite rules can flexibly constrain the permitted shapes of patches touching matching subgraphs. Second, rules can freely transform patches. We highlight the framework’s distinguishing features by comparing it against existing approaches.

# Bibtex

@inproceedings{graph:rewriting:patch:2020, author = {Overbeek, Roy and Endrullis, J{\"{o}}rg}, title = {Patch Graph Rewriting}, booktitle = {Proc.\ Conf.\ on Graph Transformation (ICGT~2020)}, series = {LNCS}, volume = {12150}, pages = {128--145}, publisher = {Springer}, year = {2020}, doi = {10.1007/978-3-030-51372-6\_8}, keywords = {rewriting}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-030-51372-6_8

## 2019

- Confluence of the Chinese MonoidJörg Endrullis, and Jan Willem KlopIn: The Art of Modelling Computational Systems, pp. 206–220, Springer (2019)paper
# Summary

The Chinese monoid, related to Knuth’s Plactic monoid, is of great interest in algebraic combinatorics. Both are ternary monoids, generated by relations between words of three symbols. The relations are, for a totally ordered alphabet, if cba = cab = bca if a ≤ b ≤ c. In this note we establish confluence by tiling for the Chinese monoid, with the consequence that every two words u, v have extensions to a common word: ∀ u,v. ∃ x,y. ux = vy.

Our proof is given using decreasing diagrams, a method for obtaining confluence that is central in abstract rewriting theory. Decreasing diagrams may also be applicable to various related monoid presentations.

We conclude with some open questions for the monoids considered.

See research for an overview of my research on confluence.

# Bibtex

@inproceedings{confluence:chinese:monoid:2019, author = {Endrullis, J{\"{o}}rg and Klop, Jan Willem}, title = {{Confluence of the Chinese Monoid}}, booktitle = {The Art of Modelling Computational Systems}, series = {LNCS}, volume = {11760}, pages = {206--220}, publisher = {Springer}, year = {2019}, doi = {10.1007/978-3-030-31175-9\_12}, keywords = {rewriting,confluence}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-030-31175-9_12

## 2018

- Decreasing Diagrams with Two Labels Are Complete for Confluence of Countable SystemsJörg Endrullis, Jan Willem Klop, and Roy OverbeekIn: Proc. Conf. on Formal Structures for Computation and Deduction (FSCD 2018), pp. 14:1–14:15, Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik (2018)paper
# Summary

The decreasing diagrams technique is one of the strongest and most versatile methods for proving confluence of abstract reduction systems. So what makes decreasing diagrams so powerful? In contrast to other confluence techniques, decreasing diagrams employ a labelling of the steps.

In this paper we investigate how the size of the label set influences the strength of the technique. Surprisingly, we find that two labels suffice for proving confluence of every confluent, countable system. In contrast, for proving commutation of rewrite relations, it turns out that the strength of the technique increases with more labels.

While decreasing diagrams is complete for proving confluence of countable systems, the technique is not complete for commutation. In our paper De Bruijn's Weak Diamond Property Revisited we give a counterexample to the completeness of decreasing diagrams for commutation.

See research for an overview of my research on confluence.

# Bibtex

@inproceedings{confluence:decreasing:diagrams:2018, author = {Endrullis, J{\"{o}}rg and Klop, Jan Willem and Overbeek, Roy}, title = {{Decreasing Diagrams with Two Labels Are Complete for Confluence of Countable Systems}}, booktitle = {Proc.\ Conf.\ on Formal Structures for Computation and Deduction (FSCD~2018)}, volume = {108}, pages = {14:1--14:15}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik}, series = {LIPIcs}, year = {2018}, doi = {10.4230/LIPIcs.FSCD.2018.14}, keywords = {rewriting, confluence}, type = {conference} }

# Digital Object Identifier

10.4230/LIPIcs.FSCD.2018.14

## 2017

- Undecidability and Finite AutomataJörg Endrullis, Jeffrey Shallit, and Tim SmithIn: Proc. Conf. Developments in Language Theory (DLT 2017), pp. 160–172, Springer (2017)paper
# Summary

Using a novel rewriting problem, we show that several natural decision problems about finite automata are undecidable. In contrast, we also prove three related problems are decidable.

We apply one result to prove the undecidability of a related problem about k-automatic sets of rational numbers.

# Bibtex

@inproceedings{automata:undecidability:2017, author = {Endrullis, J\"{o}rg and Shallit, Jeffrey and Smith, Tim}, title = {{Undecidability and Finite Automata}}, booktitle = {Proc.\ Conf.\ Developments in Language Theory (DLT~2017)}, volume = {10396}, pages = {160--172}, publisher = {Springer}, series = {LNCS}, year = {2017}, doi = {10.1007/978-3-319-62809-7\_11}, keywords = {automata, undecidability}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-319-62809-7_11

## 2016

- Degrees of Infinite Words, Polynomials and AtomsJörg Endrullis, Juhani Karhumäki, Jan Willem Klop, and Aleksi SaarelaIn: Proc. Conf. on Developments in Language Theory (DLT 2016), pp. 164–176, Springer (2016)paper
# Summary

A finite state transducer is a finite automaton that transforms input words into output words. The transducer reads the input letter by letter, in each step producing an output word and changing its state.

While finite state transducers are very simple and elegant devices, their power in transforming infinite words is hardly understood.

In this paper we show that

**techniques from continuous mathematics can be used to reason about finite state transducers**. To be precise, we use the following methods from linear algebra and analysis:- continuity,
- Vandermonde matrices,
- invertibility of matrices, and
- the generalised mean inequality.

The main result in this paper is the existance of an infinite number of atoms in the hierarchy of streams arising from finite state transduction.

We have published an extended journal version of this paper in the

*International Journal of Foundations of Computer Science*, 2018.See research for an introduction to finite state transducers, an overview of my research and many open questions.

# Bibtex

@inproceedings{streams:degrees:polynomials:2016, author = {Endrullis, J\"{o}rg and Karhum{\"{a}}ki, Juhani and Klop, Jan Willem and Saarela, Aleksi}, title = {{Degrees of Infinite Words, Polynomials and Atoms}}, booktitle = {Proc.\ Conf.\ on Developments in Language Theory (DLT~2016)}, volume = {9840}, pages = {164--176}, publisher = {Springer}, series = {LNCS}, year = {2016}, doi = {10.1007/978-3-662-53132-7\_14}, keywords = {streams, degrees, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-662-53132-7_14

## 2015

- Regularity Preserving but Not Reflecting EncodingsJörg Endrullis, Clemens Grabmayer, and Dimitri HendriksIn: Proc. Symp. on Logic in Computer Science (LICS 2015), pp. 535–546, IEEE Computer Society (2015)paper
# Summary

Encodings, that is, injective functions from words to words, have been studied extensively in several settings. In computability theory the notion of encoding is crucial for defining computability on arbitrary domains, as well as for comparing the power of models of computation. In language theory much attention has been devoted to regularity preserving functions.

A natural question arising in these contexts is: Is there a bijective encoding such that its image function preserves regularity of languages, but its pre-image function does not? Our main result answers this question in the affirmative: For every countable class \( C \) of languages there exists a bijective encoding \( f \) such that for every language \( L \in C \) its image \( f[L] \) is regular.

Our construction of such encodings has several noteworthy consequences. Firstly, anomalies arise when models of computation are compared with respect to a known concept of implementation that is based on encodings which are not required to be computable: Every countable decision model can be implemented, in this sense, by finite-state automata, even via bijective encodings. Hence deterministic finite-state automata would be equally powerful as Turing machine deciders. A second consequence concerns the recognizability of sets of natural numbers via number representations and finite automata. A set of numbers is said to be recognizable with respect to a representation if an automaton accepts the language of representations. Our result entails that there is one number representation with respect to which every recursive set is recognizable.

# Bibtex

@inproceedings{regularity:reflecting:2015, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri}, title = {{Regularity Preserving but Not Reflecting Encodings}}, booktitle = {Proc.\ Symp.\ on Logic in Computer Science (LICS~2015)}, pages = {535--546}, publisher = {{IEEE} Computer Society}, year = {2015}, doi = {10.1109/LICS.2015.56}, keywords = {automata}, type = {conference} }

# Digital Object Identifier

10.1109/LICS.2015.56

- Proving Non-Termination by Finite AutomataJörg Endrullis, and Hans ZantemaIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2015), pp. 160–176, Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik (2015)paper
# Summary

We present a new technique to prove non-termination of term rewriting. The basic idea is tofind a non-empty regular language of terms that is weakly closed under rewriting and does not containnormal forms. It is automated by representing the language by a tree automaton with a fixednumber of states, and expressing the mentioned requirements in a SAT formula. Satisfiabilityof this formula implies non-termination. Our approach succeeds for many examples where allearlier techniques fail, for instance for the S-rule from combinatory logic.

See research for an overview of my research on termination.

# Bibtex

@inproceedings{termination:automata:2015, author = {Endrullis, J\"{o}rg and Zantema, Hans}, title = {{Proving Non-termination by Finite Automata}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA~2015)}, volume = {36}, pages = {160--176}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik}, series = {LIPIcs}, year = {2015}, doi = {10.4230/LIPIcs.RTA.2015.160}, keywords = {rewriting, termination, automata}, type = {conference} }

# Digital Object Identifier

10.4230/LIPIcs.RTA.2015.160

- Degrees of TransducibilityJörg Endrullis, Jan Willem Klop, Aleksi Saarela, and Markus A. WhitelandIn: Proc. Conf. on Combinatorics on Words (WORDS 2015), pp. 1–13, Springer (2015)paper
# Summary

In this survey paper we compare different hierarchies of degrees of infinite words (streams) arising from transformation (transduction) of these words. As transformational devices we consider

- Turing machines,
- finite state transducers, and
- Mealy machines.

**degrees**, and a partial order on these degrees. We refer to these hierarchies as**Turing degrees, Transducer degrees and Mealy degrees**, respectively.While Turing degrees have been extensively studied in the 60th and 70th, hardly anything is known about Transducer and Mealy degrees. In this paper, we compare central properties of these hierarchies and mention many open problems.

See research for an introduction to finite state transducers, an overview of my research and many open questions.

# Bibtex

@inproceedings{streams:degrees:transducibility:2015, author = {Endrullis, J\"{o}rg and Klop, Jan Willem and Saarela, Aleksi and Whiteland, Markus A.}, title = {{Degrees of Transducibility}}, booktitle = {Proc.\ Conf.\ on Combinatorics on Words (WORDS~2015)}, volume = {9304}, pages = {1--13}, publisher = {Springer}, series = {LNCS}, year = {2015}, doi = {10.1007/978-3-319-23660-5\_1}, keywords = {streams, degrees, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-319-23660-5_1

- The Degree of Squares Is an AtomJörg Endrullis, Clemens Grabmayer, Dimitri Hendriks, and Hans ZantemaIn: Proc. Conf. on Combinatorics on Words (WORDS 2015), pp. 109–121, Springer (2015)paper
# Summary

A finite state transducer is a finite automaton that transforms input words into output words. The transducer reads the input letter by letter, in each step producing an output word and changing its state.

While finite state transducers are very simple and elegant devices, their power in transforming infinite words is hardly understood.

In this paper we show that the degree of the stream \[ \prod_{i = 0}^\infty 1\, 0^{i^2} = 1 \, 0^0 \, 1 \, 0^1 \, 1 \, 0^4 \, 1 \, 0^9 \, 1 \, 0^{16} \, \cdots = 1101000010000000001 \cdots \] is an atom in the hierarchy of streams arising from finite state transduction.

In the paper Degrees of Infinite Words, Polynomials and Atoms we vastly generalise this result by showing that there is precisely one atom for each polynomial degree. See research for an introduction to finite state transducers, an overview of my research and many open questions.

# Bibtex

@inproceedings{streams:degrees:squares:2015, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri and Zantema, Hans}, title = {{The Degree of Squares is an Atom}}, booktitle = {Proc.\ Conf.\ on Combinatorics on Words (WORDS~2015)}, volume = {9304}, pages = {109--121}, publisher = {Springer}, series = {LNCS}, year = {2015}, doi = {10.1007/978-3-319-23660-5\_10}, keywords = {streams, degrees, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-319-23660-5_10

- A Coinductive Framework for Infinitary Rewriting and Equational ReasoningJörg Endrullis, Helle Hvid Hansen, Dimitri Hendriks, Andrew Polonsky, and Alexandra SilvaIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2015), pp. 143–159, Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik (2015)paper
# Summary

We present a coinductive framework for defining and reasoning about the infinitary analogues of equational logic and term rewriting in a uniform, coinductive way.

The framework lends itself to an elegant and concise definition of the infinitary rewrite relation \( \to^\infty \) in terms of the single step relation \( \to \): \[ {\to^\infty} \,=\, \mu R. \nu S. ( \to \cup \mathrel{\overline{R}} )^* \mathrel{;} \overline{S} \] Here \( \mu \) and \( \nu \) are the least and greatest fixed-point operators, respectively, and \[ \overline{R} \,=\, \{\, (\, f(s_1,\ldots,s_n),\, \,f(t_1,\ldots,t_n) \,) \mid f \in \Sigma,\, s_1\! \mathrel{R} t_1,\ldots,s_n\! \mathrel{R} t_n \,\} \cup \text{Id} \] The setup captures rewrite sequences of arbitrary ordinal length, but it has neither the need for ordinals nor for metric convergence. This makes the framework suitable for formalizations in theorem provers.

We refer to Coinductive Foundations of Infinitary Rewriting and Infinitary Equational Logic (LMCS 2018) for an extended journal version of this paper.

We build on ideas in Infinitary Rewriting Coinductively (TYPES 2012) giving a coinductive perspective on infinitary lambda calculus. We extend these ideas to rewrite sequences beyond length omega by mixing induction and coinduction (least and greatest fixed-points).

# Bibtex

@inproceedings{infintary:rewriting:coinductive:2015, author = {Endrullis, J\"{o}rg and Hansen, Helle Hvid and Hendriks, Dimitri and Polonsky, Andrew and Silva, Alexandra}, title = {{A Coinductive Framework for Infinitary Rewriting and Equational Reasoning}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA 2015)}, volume = {36}, pages = {143--159}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik}, series = {LIPIcs}, year = {2015}, doi = {10.4230/LIPIcs.RTA.2015.143}, keywords = {rewriting, infinitary rewriting, coinduction}, type = {conference} }

# Digital Object Identifier

10.4230/LIPIcs.RTA.2015.143

## 2014

- Eigenvalues and Transduction of Morphic SequencesDavid Sprunger, William Tune, Jörg Endrullis, and Lawrence S. MossIn: Proc. Conf. on Developments in Language Theory (DLT 2014), pp. 239–251, Springer (2014)paper
# Summary

We study finite state transduction of automatic and morphic sequences. Dekking proved that morphic sequences are closed under transduction and in particular morphic images. We present a simple proof of this fact, and use the construction in the proof to show that non-erasing transductions preserve a condition called alpha-substitutivity. Roughly, a sequence is alpha-substitutive if the sequence can be obtained as the limit of iterating a substitution with dominant eigenvalue alpha.

Our results culminate in the following fact: for multiplicatively independent real numbers \( \alpha \) and \( \beta \), if \( v \) is an \(\alpha\)-substitutive sequence and \( w \) is a \(\beta\)-substitutive sequence, then \( v \) and \( w \) have no common non-erasing transducts except for the ultimately periodic sequences. We rely on Cobham's theorem for substitutions, a recent result of Durand.

See research for an introduction to finite state transducers, an overview of my research and many open questions.

# Bibtex

@inproceedings{streams:eigenvalues:2014, author = {Sprunger, David and Tune, William and Endrullis, J\"{o}rg and Moss, Lawrence S.}, title = {{Eigenvalues and Transduction of Morphic Sequences}}, booktitle = {Proc.\ Conf.\ on Developments in Language Theory (DLT~2014)}, volume = {8633}, pages = {239--251}, publisher = {Springer}, series = {LNCS}, year = {2014}, doi = {10.1007/978-3-319-09698-8\_21}, keywords = {streams}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-319-09698-8_21

- On Periodically Iterated MorphismsJörg Endrullis, and Dimitri HendriksIn: Joint Meeting and the Conference on Computer Science Logic (CSL) and the Symposium on Logic in Computer Science (LICS), pp. 39:1–39:10, ACM (2014)paper
# Bibtex

@inproceedings{streams:periodically:iterated:morphisms:2014, author = {Endrullis, J\"{o}rg and Hendriks, Dimitri}, title = {{On Periodically Iterated Morphisms}}, booktitle = {Joint Meeting and the Conference on Computer Science Logic (CSL) and the Symposium on Logic in Computer Science (LICS)}, pages = {39:1--39:10}, publisher = {{ACM}}, year = {2014}, doi = {10.1145/2603088.2603152}, keywords = {streams}, type = {conference} }

# Digital Object Identifier

10.1145/2603088.2603152

## 2013

- Mix-Automatic SequencesJörg Endrullis, Clemens Grabmayer, and Dimitri HendriksIn: Proc. Conf. on Language and Automata Theory and Applications (LATA 2013), pp. 262–274, Springer (2013)paper
# Bibtex

@inproceedings{streams:mix:automatic:2013, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri}, title = {{Mix-Automatic Sequences}}, booktitle = {Proc.\ Conf.\ on Language and Automata Theory and Applications (LATA~2013)}, volume = {7810}, pages = {262--274}, publisher = {Springer}, series = {LNCS}, year = {2013}, doi = {10.1007/978-3-642-37064-9\_24}, keywords = {streams, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-37064-9_24

- Circular Coinduction in Coq Using Bisimulation-Up-To TechniquesJörg Endrullis, Dimitri Hendriks, and Martin BodinIn: Proc. Conf. on Interactive Theorem Proving (ITP 2013), pp. 354–369, Springer (2013)paper
# Bibtex

@inproceedings{circular:coinduction:2013, author = {Endrullis, J\"{o}rg and Hendriks, Dimitri and Bodin, Martin}, title = {{Circular Coinduction in Coq Using Bisimulation-Up-To Techniques}}, booktitle = {Proc.\ Conf.\ on Interactive Theorem Proving (ITP~2013)}, volume = {7998}, pages = {354--369}, publisher = {Springer}, series = {LNCS}, year = {2013}, doi = {10.1007/978-3-642-39634-2\_26}, keywords = {rewriting, coinduction, formal verification}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-39634-2_26

## 2012

- Automatic Sequences and Zip-SpecificationsClemens Grabmayer, Jörg Endrullis, Dimitri Hendriks, Jan Willem Klop, and Lawrence S. MossIn: Proc. Symp. on Logic in Computer Science (LICS 2012), pp. 335–344, IEEE Computer Society (2012)paper
# Bibtex

@inproceedings{streams:zip:2012, author = {Grabmayer, Clemens and Endrullis, J\"{o}rg and Hendriks, Dimitri and Klop, Jan Willem and Moss, Lawrence S.}, title = {{Automatic Sequences and Zip-Specifications}}, booktitle = {Proc.\ Symp.\ on Logic in Computer Science (LICS~2012)}, pages = {335--344}, publisher = {{IEEE} Computer Society}, year = {2012}, doi = {10.1109/LICS.2012.44}, keywords = {rewriting, streams, automata}, type = {conference} }

# Digital Object Identifier

10.1109/LICS.2012.44

- On the Complexity of Equivalence of Specifications of Infinite ObjectsJörg Endrullis, Dimitri Hendriks, and Rena BakhshiIn: Proc. Int. Conf. on Functional Programming (ICFP 2012), pp. 153–164, ACM (2012)paper
# Bibtex

@inproceedings{complexity:stream:equality:2012, author = {Endrullis, J\"{o}rg and Hendriks, Dimitri and Bakhshi, Rena}, title = {{On the Complexity of Equivalence of Specifications of Infinite Objects}}, booktitle = {Proc.\ Int.\ Conf.\ on Functional Programming (ICFP~2012)}, pages = {153--164}, publisher = {{ACM}}, year = {2012}, doi = {10.1145/2364527.2364551}, keywords = {rewriting, undecidability, streams}, type = {conference} }

# Digital Object Identifier

10.1145/2364527.2364551

## 2011

- Proving Equality of Streams AutomaticallyHans Zantema, and Jörg EndrullisIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2011), pp. 393–408, Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik (2011)paper
# Bibtex

@inproceedings{streams:equality:2011, author = {Zantema, Hans and Endrullis, J\"{o}rg}, title = {{Proving Equality of Streams Automatically}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA~2011)}, volume = {10}, pages = {393--408}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik}, series = {LIPIcs}, year = {2011}, doi = {10.4230/LIPIcs.RTA.2011.393}, keywords = {rewriting, infinitary rewriting, streams}, type = {conference} }

# Digital Object Identifier

10.4230/LIPIcs.RTA.2011.393

- Infinitary Rewriting CoinductivelyJörg Endrullis, and Andrew PolonskyIn: Proc. Conf. on Types for Proofs and Programs (TYPES 2012), pp. 16–27, Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik (2011)paper
# Summary

We provide a coinductive definition of strongly convergent reductions between infinite lambda terms. This approach avoids the notions of ordinals and metric convergence which have appeared in the earlier definitions of the concept. As an illustration, we prove the existence part of the infinitary standardization theorem. The proof is fully formalized in Coq using coinductive types.

The paper concludes with a characterization of infinite lambda terms which reduce to themselves in a single beta step.

The papers

extend the idea in this paper to reductions of length beyond omega.The extension to reduction lengths beyond omega requires mixing of induction and coinduction. Therefore the simpler setup presented in the current paper (

*Infinitary Rewriting Coinductively*) is preferrable whenever there is no need for reductions longer than omega. This concerns for instance rewrite systems having the compression property, such as lambda calculus and left-linear term rewriting systems (with finite left-hand sides).# Bibtex

@inproceedings{infintary:lambda:coinductive:2011, author = {Endrullis, J\"{o}rg and Polonsky, Andrew}, title = {{Infinitary Rewriting Coinductively}}, booktitle = {Proc.\ Conf.\ on Types for Proofs and Programs (TYPES~2012)}, volume = {19}, pages = {16--27}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik}, series = {LIPIcs}, year = {2011}, doi = {10.4230/LIPIcs.TYPES.2011.16}, keywords = {rewriting, infinitary rewriting, coinduction}, type = {conference} }

# Digital Object Identifier

10.4230/LIPIcs.TYPES.2011.16

## 2010

- Brief Announcement: Asynchronous Bounded Expected Delay NetworksRena Bakhshi, Jörg Endrullis, Wan Fokkink, and Jun PangIn: Proc. Symp. on Principles of Distributed Computing (PODC 2010), pp. 392–393, ACM (2010)paper
# Bibtex

@inproceedings{bounded:expected:delay:2010, author = {Bakhshi, Rena and Endrullis, J\"{o}rg and Fokkink, Wan and Pang, Jun}, title = {{Brief Announcement: Asynchronous Bounded Expected Delay Networks}}, booktitle = {Proc.\ Symp.\ on Principles of Distributed Computing (PODC~2010)}, pages = {392--393}, publisher = {{ACM}}, year = {2010}, doi = {10.1145/1835698.1835787}, keywords = {protocols}, type = {conference} }

# Digital Object Identifier

10.1145/1835698.1835787

- Automating the Mean-Field Method for Large Dynamic Gossip NetworksRena Bakhshi, Jörg Endrullis, Stefan Endrullis, Wan Fokkink, and Boudewijn R. HaverkortIn: Proc. Conf. on Quantitative Evaluation of Systems (QEST 2010), pp. 241–250, IEEE Computer Society (2010)paper
# Bibtex

@inproceedings{meanfield:2010, author = {Bakhshi, Rena and Endrullis, J\"{o}rg and Endrullis, Stefan and Fokkink, Wan and Haverkort, Boudewijn R.}, title = {{Automating the Mean-Field Method for Large Dynamic Gossip Networks}}, booktitle = {Proc.\ Conf.\ on Quantitative Evaluation of Systems (QEST~2010)}, pages = {241--250}, publisher = {{{IEEE} Computer Society}}, year = {2010}, doi = {10.1109/QEST.2010.38}, keywords = {protocols}, type = {conference} }

# Digital Object Identifier

10.1109/QEST.2010.38

- Modular Construction of Fixed Point Combinators and Clocked Böhm TreesJörg Endrullis, Dimitri Hendriks, and Jan Willem KlopIn: Proc. Symp. on Logic in Computer Science (LICS 2010), pp. 111–119, IEEE Computer Society (2010)paper
# Summary

One of the best-known methods for discriminating lambda terms with respect to \( \beta \)-convertibility is due to Corrado Böhm. Roughly speaking, the

*Böhm tree*of a lambda term is its infinite normal form. If lambda terms have distinct Böhm trees, then they are not \( \beta \)-convertible. But what if their Böhm trees coincide? For example, all fixed-point combinators have the same Böhm tree, namely \[ \lambda x. x(x(x(\ldots))) \]We refine Böhm trees with a

*clock*that records how many head reduction steps have been used to rewrite each subterm to head normal form. We show that*clocked Böhm trees*are suitable for discriminating a rich class of lambda terms (in particular many fixed-point combinators) having the same Böhm trees.We refer to

*Discriminating Lambda-Terms Using Clocked Boehm Trees*(LMCS 2014) for an extended journal version of this paper. In the extended version we improve the precision of the clocks to*atomic clocks*and we answer a question of Gordon Plotkin, showing that the method can be used beyond simple terms.See research for an overview of my research on the clocked lambda claculus and fixed-point combinators.

# Bibtex

@inproceedings{lambda:clocks:2010, author = {Endrullis, J\"{o}rg and Hendriks, Dimitri and Klop, Jan Willem}, title = {{Modular Construction of Fixed Point Combinators and Clocked B\"{o}hm Trees}}, booktitle = {Proc.\ Symp.\ on Logic in Computer Science (LICS~2010)}, pages = {111--119}, publisher = {{IEEE} Computer Society}, year = {2010}, doi = {10.1109/LICS.2010.8}, keywords = {rewriting, infinitary rewriting, lambda calculus}, type = {conference} }

# Digital Object Identifier

10.1109/LICS.2010.8

- Unique Normal Forms in Infinitary Weakly Orthogonal RewritingJörg Endrullis, Clemens Grabmayer, Dimitri Hendriks, Jan Willem Klop, and Vincent van OostromIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2010), pp. 85–102, Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik (2010)paper
# Bibtex

@inproceedings{infinitary:weakly:orthogonal:unique:normal:forms:2010, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri and Klop, Jan Willem and van~Oostrom, Vincent}, title = {{Unique Normal Forms in Infinitary Weakly Orthogonal Rewriting}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA~2010)}, volume = {6}, pages = {85--102}, publisher = {Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik}, series = {LIPIcs}, year = {2010}, doi = {10.4230/LIPIcs.RTA.2010.85}, keywords = {rewriting, infinitary rewriting, lambda calculus}, type = {conference} }

# Digital Object Identifier

10.4230/LIPIcs.RTA.2010.85

## 2009

- From Outermost to Context-Sensitive RewritingJörg Endrullis, and Dimitri HendriksIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2009), pp. 305–319, Springer (2009)paper
# Summary

We define a transformation from term rewriting systems (TRSs) to context-sensitive TRSs in such a way that termination of the target system implies outermost termination of the original system. For the class of left-linear TRSs the transformation is complete. Thereby state-of-the-art termination methods and automated termination provers for context-sensitive rewriting become available for proving termination of outermost rewriting. The translation has been implemented in Jambox, making it the most successful tool in the category of outermost rewriting of the last edition of the annual termination competition.

An extended version of this paper is available from Transforming Outermost into Context-Sensitive Rewriting (Logical Methods in Computer Science 2010).

See research for an overview of my research on termination.

# Bibtex

@inproceedings{termination:outermost:2009, author = {Endrullis, J\"{o}rg and Hendriks, Dimitri}, title = {{From Outermost to Context-Sensitive Rewriting}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA~2009)}, volume = {5595}, pages = {305--319}, publisher = {Springer}, series = {LNCS}, year = {2009}, doi = {10.1007/978-3-642-02348-4\_22}, keywords = {rewriting, termination, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-02348-4_22

- Local TerminationJörg Endrullis, Roel C. de Vrijer, and Johannes WaldmannIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2009), pp. 270–284, Springer (2009)paper
# Bibtex

@inproceedings{termination:local:2009, author = {Endrullis, J\"{o}rg and de~Vrijer, Roel C. and Waldmann, Johannes}, title = {{Local Termination}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA~2009)}, volume = {5595}, pages = {270--284}, publisher = {Springer}, series = {LNCS}, year = {2009}, doi = {10.1007/978-3-642-02348-4\_19}, keywords = {rewriting, termination, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-02348-4_19

- Complexity of Fractran and ProductivityJörg Endrullis, Clemens Grabmayer, and Dimitri HendriksIn: Proc. Conf. on Automated Deduction (CADE 2009), pp. 371–387, Springer (2009)paper
# Bibtex

@inproceedings{complexity:productivity:2009, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri}, title = {{Complexity of Fractran and Productivity}}, booktitle = {Proc.\ Conf.\ on Automated Deduction (CADE~2009)}, volume = {5663}, pages = {371--387}, publisher = {Springer}, series = {LNCS}, year = {2009}, doi = {10.1007/978-3-642-02959-2\_28}, keywords = {rewriting, undecidability, productivity}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-02959-2_28

- Degrees of Undecidability in Term RewritingJörg Endrullis, Herman Geuvers, and Hans ZantemaIn: Proc. Conf. on Computer Science Logic (CSL 2009), pp. 255–270, Springer (2009)paper
# Summary

Undecidability of various properties of first-order term rewriting systems is well-known. An undecidable property can be classified by the complexity of the formula defining it. This classification gives rise to a hierarchy of distinct levels of undecidability, starting from the arithmetical hierarchy classifying properties using first order arithmetical formulas, and continuing into the analytic hierarchy, where quantification over function variables is allowed.

In this paper we give an overview of how the main properties of first-order term rewriting systems are classified in these hierarchies. We consider properties related to normalization (strong normalization, weak normalization and dependency problems) and properties related to confluence (confluence, local confluence and the unique normal form property). For all of these we distinguish between the single term version and the uniform version. Where appropriate, we also distinguish between ground and open terms.

Most uniform properties are \( \Pi^0_2 \)-complete. The particular problem of local confluence turns out to be \( \Pi^0_2 \)-complete for ground terms, but only \( \Sigma^0_1 \)-complete (and thereby recursively enumerable) for open terms. The most surprising result concerns dependency pair problems without minimality flag: we prove this problem to be \( \Pi^1_1 \)-complete, hence not in the arithmetical hierarchy, but properly in the analytic hierarchy.

We refer to

*Levels of Undecidability in Rewriting*(Information and Computation 2011) for an extended journal version of this paper.# Bibtex

@inproceedings{rewriting:undecidability:degrees:2009, author = {Endrullis, J\"{o}rg and Geuvers, Herman and Zantema, Hans}, title = {{Degrees of Undecidability in Term Rewriting}}, booktitle = {Proc.\ Conf.\ on Computer Science Logic (CSL~2009)}, volume = {5771}, pages = {255--270}, publisher = {Springer}, series = {LNCS}, year = {2009}, doi = {10.1007/978-3-642-04027-6\_20}, keywords = {rewriting, undecidability, termination, confluence}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-04027-6_20

## 2008

- Data-Oblivious Stream ProductivityJörg Endrullis, Clemens Grabmayer, and Dimitri HendriksIn: Proc. Conf. on Logic for Programming Artificial Intelligence and Reasoning (LPAR 2008), pp. 79–96, Springer (2008)paper
# Summary

We are concerned with demonstrating productivity of specifications of infinite streams of data, based on orthogonal rewrite rules. In general, this property is undecidable, but for restricted formats computable sufficient conditions can be obtained. The usual analysis, also adopted here, disregards the identity of data, thus leading to approaches that we call data-oblivious. We present a method that is provably optimal among all such data-oblivious approaches. This means that in order to improve on our algorithm one has to proceed in a data-aware fashion.

See research for an overview of my research on productivity.

# Bibtex

@inproceedings{productivity:data:oblivious:2008, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri}, title = {{Data-Oblivious Stream Productivity}}, booktitle = {Proc.\ Conf.\ on Logic for Programming Artificial Intelligence and Reasoning (LPAR~2008)}, number = {5330}, pages = {79--96}, publisher = {Springer}, series = {LNCS}, year = {2008}, doi = {10.1007/978-3-540-89439-1\_6}, keywords = {rewriting, infinitary rewriting, productivity}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-540-89439-1_6

- Reduction under SubstitutionJörg Endrullis, and Roel C. de VrijerIn: Proc. Conf. on Rewriting Techniques and Applications (RTA 2008), pp. 425–440, Springer (2008)paper
# Bibtex

@inproceedings{lambda:reduction:under:substitution:2008, author = {Endrullis, J\"{o}rg and de~Vrijer, Roel C.}, title = {{Reduction under Substitution}}, booktitle = {Proc.\ Conf.\ on Rewriting Techniques and Applications (RTA~2008)}, volume = {5117}, pages = {425--440}, publisher = {Springer}, series = {LNCS}, year = {2008}, doi = {10.1007/978-3-540-70590-1\_29}, keywords = {rewriting, lambda calculus}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-540-70590-1_29

- Proving Infinitary NormalizationJörg Endrullis, Clemens Grabmayer, Dimitri Hendriks, Jan Willem Klop, and Roel C. de VrijerIn: Proc. Conf. on Types for Proofs and Programs (TYPES 2008), pp. 64–82, Springer (2008)paper
# Bibtex

@inproceedings{infinitary:normalization:2009, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri and Klop, Jan Willem and de~Vrijer, Roel C.}, title = {{Proving Infinitary Normalization}}, booktitle = {Proc.\ Conf.\ on Types for Proofs and Programs (TYPES~2008)}, volume = {5497}, pages = {64--82}, publisher = {Springer}, series = {LNCS}, year = {2008}, doi = {10.1007/978-3-642-02444-3\_5}, keywords = {rewriting, infinitary rewriting, automata}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-642-02444-3_5

## 2007

- Productivity of Stream DefinitionsJörg Endrullis, Clemens Grabmayer, Dimitri Hendriks, Ariya Isihara, and Jan Willem KlopIn: Proc. Symp. on Fundamentals of Computation Theory (FCT 2007), pp. 274–287, Springer (2007)paper
# Summary

We give an algorithm for deciding productivity of a large and natural class of recursive stream definitions. A stream definition is called `productive' if it can be evaluated continually in such a way that a uniquely determined stream in constructor normal form is obtained as the limit. Whereas productivity is undecidable for stream definitions in general, we show that it can be decided for `pure' stream definitions. For every pure stream definition the process of its evaluation can be modelled by the dataflow of abstract stream elements, called `pebbles', in a finite `pebbleflow net(work)'. And the production of a pebbleflow net associated with a pure stream definition, that is, the amount of pebbles the net is able to produce at its output port, can be calculated by reducing nets to trivial nets.

An extended journal version of this paper is available from Productivity of Stream Definitions (TCS 2010).

See research for an overview of my research on productivity.

# Bibtex

@inproceedings{productivity:streams:2007, author = {Endrullis, J\"{o}rg and Grabmayer, Clemens and Hendriks, Dimitri and Isihara, Ariya and Klop, Jan Willem}, title = {{Productivity of Stream Definitions}}, booktitle = {Proc.\ Symp.\ on Fundamentals of Computation Theory (FCT~2007)}, number = {4639}, pages = {274--287}, publisher = {Springer}, series = {LNCS}, year = {2007}, doi = {10.1007/978-3-540-74240-1\_24}, keywords = {rewriting, infinitary rewriting, productivity}, type = {conference} }

# Digital Object Identifier

10.1007/978-3-540-74240-1_24

## 2006

- Matrix Interpretations for Proving Termination of Term RewritingJörg Endrullis, Johannes Waldmann, and Hans ZantemaIn: Proc. Int. Joint Conf. on Automated Reasoning (IJCAR 2006), pp. 574–588, Springer (2006)paper
# Summary

We present a new method for automatically proving termination of term rewriting using matrix interpretation (weighted automata with natural numbers as weights). It is based on the well-known idea of interpretation of terms where every rewrite step causes a decrease, but instead of the usual natural numbers we use vectors of natural numbers, ordered by a particular non-total well-founded ordering. Function symbols are interpreted by linear mappings represented by matrices. This method allows to prove termination and relative termination. A modification of the latter in which strict steps are only allowed at the top, turns out to be helpful in combination with the dependency pair transformation.

By bounding the dimension and the matrix coefficients, the search problem becomes finite. Our implementation transforms it to a Boolean satisfiability problem (SAT), to be solved by a state-of-the-art SAT solver. Our implementation performs well on the Termination Problem Data Base: better than 5 out of 6 tools that participated in the 2005 termination competition in the category of term rewriting.

An extended journal version of this paper is available from Matrix Interpretations for Proving Termination of Term Rewriting (Journal of Automated Reasoning 2008).

See research for an overview of my research on termination.

# Bibtex

@inproceedings{termination:matrix:2006, author = {Endrullis, J\"{o}rg and Waldmann, Johannes and Zantema, Hans}, title = {{Matrix Interpretations for Proving Termination of Term Rewriting}}, booktitle = {Proc.\ Int.\ Joint Conf.\ on Automated Reasoning (IJCAR~2006)}, volume = {4130}, pages = {574--588}, publisher = {Springer}, series = {LNCS}, year = {2006}, doi = {10.1007/11814771\_47}, keywords = {rewriting, termination, automata}, type = {conference} }

# Digital Object Identifier

10.1007/11814771_47