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Henri Hansen

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Krishnendu Rarhi

Chandigarh University Gharuan

Yeungnam University

Dr. Pramod Kumar

Assam Downtown University

Felicita Chromjaková

Tomas Bata University of Zlín

İbrahim Dursun

Ravindra Hewa Kuruppuge

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Papers by Henri Hansen

Retaliation in Bitcoin networks

Economics Letters, Jun 1, 2021

Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust ... more Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust they have in others. We examine users' feedback behavior on two Bitcoin trading platforms and provide statistically strong evidence that the feedback behavior of Bitcoin users is dependent on how they are rated themselves, that is, they retaliate. In addition, user's reputation is strongly and positively associated with the scores they deliver, and there is a certain persistence in the scores a user gives to others. We find that peers deliver negative feedback relatively quickly to users with bad reputation. Moreover, well-reputed users withhold negative feedback longer and give positive feedback faster than users with bad reputation.

Minimal Counterexamples in O(n log n) Memory and O(n^2) Time

Operational Determinism and Fast Algorithms

Springer eBooks, 2006

The main contribution of this work is a fast algorithm for checking whether a labelled transition... more The main contribution of this work is a fast algorithm for checking whether a labelled transition system (LTS) is operationally deterministic. Operational determinism is a condition on the LTS designed to capture the notion of “deterministic behaviour” without ruling out invisible actions and divergence, and without strictly devoting oneself to any single process-algebraic semantics. Indeed, we show that in the case of operationally deterministic LTSs, all divergence-sensitive equivalences between divergence-sensitive branching bisimilarity and trace + divergence trace equivalence collapse to the same equivalence. The running time of the algorithm is linear except a term that, roughly speaking, grows as slowly as Ackermann’s function grows quickly. If the original LTS is operationally deterministic, the algorithm produces as a by-product a structurally deterministic LTS that is divergence-sensitive branching bisimilar to the original one. This LTS can be minimised like a deterministic finite automaton. The overall approach is so cheap that it makes almost always sense to first try it and revert to a semantics-specific reduction or minimisation algorithm only if the LTS proves operationally nondeterministic.

Partial Order Reduction for Model Checking Markov Decision Processes under Unconditional Fairness

Fairness assumptions are needed to verify liveness properties of concurrent systems. In this pape... more Fairness assumptions are needed to verify liveness properties of concurrent systems. In this paper we explore the so-called unconditional fairness in Markov decision processes (MDPs), which is a prerequisite for quantitative assumeguarantee reasoning. Unconditional fairness refers to executions where all processes are guaranteed to participate. We prove that realisability of unconditional fairness coincides with the absence of partial deadlocks, i.e., end components where a process suffers from starvation. We propose a weak variant of the stubborn set method to reduce MDPs, while preserving the realisability of unconditional fairness and maximal probabilities of reaching bottom end components under fair schedulers.

Stubborn Set Intuition Explained

Lecture Notes in Computer Science, 2017

This study focuses on the differences between stubborn sets and other partial order methods. Firs... more This study focuses on the differences between stubborn sets and other partial order methods. First a major problem with step graphs is pointed out with an example. Then the deadlock-preserving stubborn set method is compared to the deadlock-preserving ample set and persistent set methods. Next, conditions are discussed whose purpose is to ensure that the reduced state space preserves the ordering of visible transitions, that is, transitions that may change the truth values of the propositions that the formula under verification has been built from. Finally solutions to the ignoring problem are analysed both when the purpose is to preserve only safety properties and when also liveness properties are of interest.

Tampere Verification Tool

Springer eBooks, 2004

Tampere Verification Tool (TVT) is a collection of programs for automated verification of concurr... more Tampere Verification Tool (TVT) is a collection of programs for automated verification of concurrent and reactive systems. TVT has its roots in process algebras and explicit state space exploration, but in addition to actions, our formalism allows use of state-based information in the form of truth-valued state propositions. Furthermore, it contains three types of state proposition-like notions to support on-the-fly verification, and one state proposition to exploit partially defined processes. TVT supports compositional state space construction, stubborn sets and visual verification.

On the Origin of Events: Branching Cells as Stubborn Sets

Lecture Notes in Computer Science, 2011

In prime event structures with binary conflicts (pes-bc) 3 a branching cell [1] is a subset of ev... more In prime event structures with binary conflicts (pes-bc) 3 a branching cell [1] is a subset of events closed under downward causality and immediate conflict relations. This means that no event outside the branching cell can be in conflict with or enable any event inside the branching cell. It bears a strong resemblance to stubborn sets, a partial order reduction method on transition systems. A stubborn set (at a given state) is a subset of actions such that no execution consisting entirely of actions outside the stubborn set can be in conflict with or enable actions that are inside the stubborn set. A rigorous study of the relationship between the two ideas, however, is not straightforward due to the facts that 1) stubborn sets utilise sophisticated causality and conflict relations that invalidate the stability and coherence of event structures [18], 2) without stability it becomes very difficult to define concepts like prefixes and branching cells, which prerequire a clear notion of causality, and 3) it is challenging to devise a technique for identifying 'proper' subsets of transitions as 'events' such that the induced event-based system captures exactly the causality and conflict information needed by stubborn sets. In this paper we give a solution to the problems by providing an unfolding of labelled transition systems into configuration structures, a more general structure supporting or-causality and finite conflict. We show that the branching cell definition can be extended to configuration structures and that each branching cell in the unfolding is a long-lived stubborn set, such that no matter how the system evolves, what remains of the branching cell is always a stubborn set.

Exploring the Scope for Partial Order Reduction

Lecture Notes in Computer Science, 2009

Can Stubborn Sets Be Optimal?

Lecture Notes in Computer Science, 2010

Merging state-based and action-based verification

A formalism is presented that is intended to combine basic properties of both state-based and act... more A formalism is presented that is intended to combine basic properties of both state-based and action-based verification. In state-based verification the behaviour of the system is described in terms of the properties of its states, whereas action-based methods concentrate on transitions between states. A typical state-based approach consists of representing requirements as temporal logic formulae, and model-checking the state space of the system against them. Action-based verification often consists of comparing systems according to some equivalence or preorder relation. We add state propositions to a typical process-algebraic action framework. Values of state propositions are propagated through process-algebraic compositions and reductions by augmenting actions with changes of proposition values. A modified parallel composition operator is used for synchronisation of processes and handling of state propositions. Efficient on-the-fly verification is obtained with four kinds of rejection conditions. The formalism is implemented in a new verification tool TVT.

Larger Automata and Less Work for LTL Model Checking

Lecture Notes in Computer Science, 2006

Retaliation in Bitcoin Networks

Social Science Research Network, 2021

Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust ... more Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust they have in others. We examine users' feedback behaviour on two Bitcoin trading platforms and provide statistically strong evidence that the feedback behaviour of Bitcoin users is dependent on how they are rated themselves, that is, they retaliate. In addition, user's reputation is strongly and positively associated with the scores they deliver, and there is a certain persistence in the scores a user gives to others. We find that peers deliver negative feedback relatively quickly to users with bad reputation. Moreover, well-reputed users withhold negative feedback longer and give positive feedback faster than users with bad reputation.

Abstractions for Transition Systems with Applications to Stubborn Sets

Lecture Notes in Computer Science, Dec 18, 2016

Partial order reduction covers a range of techniques based on eliminating unnecessary transitions... more Partial order reduction covers a range of techniques based on eliminating unnecessary transitions when generating a state space. On the other hand, abstractions replace sets of states of a system with abstract representatives in order to create a smaller state space. This article explores how stubborn sets and abstraction can be combined. We provide examples to provide intuition and expand on some recent results. We provide a classification of abstractions and give some novel results on what is needed to combine abstraction and partial order reduction in a sound way.

Safety Property-Driven Stubborn Sets

Lecture Notes in Computer Science, 2016

A new reduced state space construction method is presented where in every constructed state, the ... more A new reduced state space construction method is presented where in every constructed state, the set of transitions that are fired is chosen based on the safety property that is being verified. Typical earlier methods only take the property into account in one state of each cycle or in one state of each terminal strong component of the reduced state space. They may fire totally irrelevant transitions in the other states. Where the property is taken into account, typically many or all enabled transitions are fired. This has spoiled attempts to be property-driven in every state. The present study exploits an idea that was published in 2016 with which this can be avoided. Furthermore, most earlier methods classify the transitions to visible and invisible. The new method uses a novel improved concept. An experiment is presented where the new concept provides significant improvement to the reduction results.

Compositional Analysis for Weak Stubborn Sets

Partial order reduction methods rely on expanding a state space only partially, exploring represe... more Partial order reduction methods rely on expanding a state space only partially, exploring representatives of sets of executions of a system. The methods differ at least in three respects:the set of properties that are preserved, the notions of interaction between transitions, and the methods of selecting the representative transitions. We explore an existing but less studied method of weak stubborn sets in the context of process-algebraic parallel composition. The theory of stubborn set methods is based on global condition son executions. In practice, these conditions are guaranteed by using static dependency information in the form of dependency and causality relations. We propose a compositional approach for these conditions. We extract dependency information from the component processes by analysing the components in detail and then define compositional rules for system-level dependency information. We use novel localised conditions of dependency that the weak stubborn set method can make use of. We carry out some experiments in the context of FDR, to explore the relative merits of the method compared to the more well-known version of stubborn sets with and without compositional analysis. We do this for both a deadlock preserving reduction and a version that preserves the failures/divergences semantics of FDR.

Old and New Algorithms for Minimal Coverability Sets

Fundamenta Informaticae, 2014

Many algorithms for computing minimal coverability sets for Petri nets prune futures. That is, if... more Many algorithms for computing minimal coverability sets for Petri nets prune futures. That is, if a newmarking strictly covers an old one, then not just the old marking but also some subset of its successor markings is discarded from search. In this publication, a simpler algorithm that lacks future pruning is presented and proven correct. Its performance is compared with future pruning. It is demonstrated, using examples, that neither approach is systematically better than the other. However, the simple algorithm has some attractive features. It never needs to re-construct pruned parts of the minimal coverability set. It automatically gives most of the advantage of future pruning, if the minimal coverability set is constructed in depth-first or most tokens first order, and if so-called history merging is applied. Some implementation aspects of minimal coverability set construction are also discussed. Some measurements are given to demonstrate the effect of construction order and other implementation aspects.

Partial order reduction for model checking Markov decision processes under unconditional

Fairness assumptions are needed to verify liveness properties of concurrent systems. In this pape... more Fairness assumptions are needed to verify liveness properties of concurrent systems. In this paper we explore the so-called unconditional fairness in Markov decision processes (MDPs), which is a prerequisite for quantitative assumeguarantee reasoning. Unconditional fairness refers to executions where all processes are guaranteed to participate. We prove that realisability of unconditional fairness coincides with the absence of partial deadlocks, i.e., end components where a process suffers from starvation. We propose a weak variant of the stubborn set method to reduce MDPs, while preserving the realisability of unconditional fairness and maximal probabilities of reaching bottom end components under fair schedulers.

Oski Kervinen: An Architecture for Verification with Ex- Tended Labeled Transition Systems

The examiners and topic of the thesis were approved by the Council of the Fac-

Why Confluence is More Powerful than Ample Sets in Probabilistic and Non-Probabilistic Branching Time

Confluence reduction and partial order reduction by means of ample sets are two different techniq... more Confluence reduction and partial order reduction by means of ample sets are two different techniques for state space reduction in both traditional and probabilistic model checking. This presentation provides an extensive comparison between these two methods, answering the long-standing question of how they relate. We show that, while both preserve branching time properties, confluence reduction is strictly more powerful than partial order reduction: every reduction that can be obtained with partial order reduction can also be obtained with confluence reduction, but the converse is not true. A core problem in the comparison is that confluence reduction was defined in an action-based setting, whereas partial order reduction was defined in a state-based setting. We therefore redefine confluence reduction in the state-based setting of Markov decision processes, and discuss a nontrivial proof of its correctness. Additionally, we pinpoint precisely in what way confluence reduction is more...

Teemu Stenhammar Factorization of Binary Polynomials

TEEMU STENHAMMAR: Factorization of binary polynomials Tampere University of Technology Master of ... more

Retaliation in Bitcoin networks

Economics Letters, Jun 1, 2021

Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust ... more Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust they have in others. We examine users' feedback behavior on two Bitcoin trading platforms and provide statistically strong evidence that the feedback behavior of Bitcoin users is dependent on how they are rated themselves, that is, they retaliate. In addition, user's reputation is strongly and positively associated with the scores they deliver, and there is a certain persistence in the scores a user gives to others. We find that peers deliver negative feedback relatively quickly to users with bad reputation. Moreover, well-reputed users withhold negative feedback longer and give positive feedback faster than users with bad reputation.

Minimal Counterexamples in O(n log n) Memory and O(n^2) Time

Operational Determinism and Fast Algorithms

Springer eBooks, 2006

The main contribution of this work is a fast algorithm for checking whether a labelled transition... more The main contribution of this work is a fast algorithm for checking whether a labelled transition system (LTS) is operationally deterministic. Operational determinism is a condition on the LTS designed to capture the notion of “deterministic behaviour” without ruling out invisible actions and divergence, and without strictly devoting oneself to any single process-algebraic semantics. Indeed, we show that in the case of operationally deterministic LTSs, all divergence-sensitive equivalences between divergence-sensitive branching bisimilarity and trace + divergence trace equivalence collapse to the same equivalence. The running time of the algorithm is linear except a term that, roughly speaking, grows as slowly as Ackermann’s function grows quickly. If the original LTS is operationally deterministic, the algorithm produces as a by-product a structurally deterministic LTS that is divergence-sensitive branching bisimilar to the original one. This LTS can be minimised like a deterministic finite automaton. The overall approach is so cheap that it makes almost always sense to first try it and revert to a semantics-specific reduction or minimisation algorithm only if the LTS proves operationally nondeterministic.

Partial Order Reduction for Model Checking Markov Decision Processes under Unconditional Fairness

Fairness assumptions are needed to verify liveness properties of concurrent systems. In this pape... more Fairness assumptions are needed to verify liveness properties of concurrent systems. In this paper we explore the so-called unconditional fairness in Markov decision processes (MDPs), which is a prerequisite for quantitative assumeguarantee reasoning. Unconditional fairness refers to executions where all processes are guaranteed to participate. We prove that realisability of unconditional fairness coincides with the absence of partial deadlocks, i.e., end components where a process suffers from starvation. We propose a weak variant of the stubborn set method to reduce MDPs, while preserving the realisability of unconditional fairness and maximal probabilities of reaching bottom end components under fair schedulers.

Stubborn Set Intuition Explained

Lecture Notes in Computer Science, 2017

This study focuses on the differences between stubborn sets and other partial order methods. Firs... more This study focuses on the differences between stubborn sets and other partial order methods. First a major problem with step graphs is pointed out with an example. Then the deadlock-preserving stubborn set method is compared to the deadlock-preserving ample set and persistent set methods. Next, conditions are discussed whose purpose is to ensure that the reduced state space preserves the ordering of visible transitions, that is, transitions that may change the truth values of the propositions that the formula under verification has been built from. Finally solutions to the ignoring problem are analysed both when the purpose is to preserve only safety properties and when also liveness properties are of interest.

Tampere Verification Tool

Springer eBooks, 2004

Tampere Verification Tool (TVT) is a collection of programs for automated verification of concurr... more Tampere Verification Tool (TVT) is a collection of programs for automated verification of concurrent and reactive systems. TVT has its roots in process algebras and explicit state space exploration, but in addition to actions, our formalism allows use of state-based information in the form of truth-valued state propositions. Furthermore, it contains three types of state proposition-like notions to support on-the-fly verification, and one state proposition to exploit partially defined processes. TVT supports compositional state space construction, stubborn sets and visual verification.

On the Origin of Events: Branching Cells as Stubborn Sets

Lecture Notes in Computer Science, 2011

In prime event structures with binary conflicts (pes-bc) 3 a branching cell [1] is a subset of ev... more In prime event structures with binary conflicts (pes-bc) 3 a branching cell [1] is a subset of events closed under downward causality and immediate conflict relations. This means that no event outside the branching cell can be in conflict with or enable any event inside the branching cell. It bears a strong resemblance to stubborn sets, a partial order reduction method on transition systems. A stubborn set (at a given state) is a subset of actions such that no execution consisting entirely of actions outside the stubborn set can be in conflict with or enable actions that are inside the stubborn set. A rigorous study of the relationship between the two ideas, however, is not straightforward due to the facts that 1) stubborn sets utilise sophisticated causality and conflict relations that invalidate the stability and coherence of event structures [18], 2) without stability it becomes very difficult to define concepts like prefixes and branching cells, which prerequire a clear notion of causality, and 3) it is challenging to devise a technique for identifying 'proper' subsets of transitions as 'events' such that the induced event-based system captures exactly the causality and conflict information needed by stubborn sets. In this paper we give a solution to the problems by providing an unfolding of labelled transition systems into configuration structures, a more general structure supporting or-causality and finite conflict. We show that the branching cell definition can be extended to configuration structures and that each branching cell in the unfolding is a long-lived stubborn set, such that no matter how the system evolves, what remains of the branching cell is always a stubborn set.

Exploring the Scope for Partial Order Reduction

Lecture Notes in Computer Science, 2009

Can Stubborn Sets Be Optimal?

Lecture Notes in Computer Science, 2010

Merging state-based and action-based verification

A formalism is presented that is intended to combine basic properties of both state-based and act... more A formalism is presented that is intended to combine basic properties of both state-based and action-based verification. In state-based verification the behaviour of the system is described in terms of the properties of its states, whereas action-based methods concentrate on transitions between states. A typical state-based approach consists of representing requirements as temporal logic formulae, and model-checking the state space of the system against them. Action-based verification often consists of comparing systems according to some equivalence or preorder relation. We add state propositions to a typical process-algebraic action framework. Values of state propositions are propagated through process-algebraic compositions and reductions by augmenting actions with changes of proposition values. A modified parallel composition operator is used for synchronisation of processes and handling of state propositions. Efficient on-the-fly verification is obtained with four kinds of rejection conditions. The formalism is implemented in a new verification tool TVT.

Larger Automata and Less Work for LTL Model Checking

Lecture Notes in Computer Science, 2006

Retaliation in Bitcoin Networks

Social Science Research Network, 2021

Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust ... more Due to counterparty risks, some Bitcoin trading platforms allow users to rate the level of trust they have in others. We examine users' feedback behaviour on two Bitcoin trading platforms and provide statistically strong evidence that the feedback behaviour of Bitcoin users is dependent on how they are rated themselves, that is, they retaliate. In addition, user's reputation is strongly and positively associated with the scores they deliver, and there is a certain persistence in the scores a user gives to others. We find that peers deliver negative feedback relatively quickly to users with bad reputation. Moreover, well-reputed users withhold negative feedback longer and give positive feedback faster than users with bad reputation.

Abstractions for Transition Systems with Applications to Stubborn Sets

Lecture Notes in Computer Science, Dec 18, 2016

Partial order reduction covers a range of techniques based on eliminating unnecessary transitions... more Partial order reduction covers a range of techniques based on eliminating unnecessary transitions when generating a state space. On the other hand, abstractions replace sets of states of a system with abstract representatives in order to create a smaller state space. This article explores how stubborn sets and abstraction can be combined. We provide examples to provide intuition and expand on some recent results. We provide a classification of abstractions and give some novel results on what is needed to combine abstraction and partial order reduction in a sound way.

Safety Property-Driven Stubborn Sets

Lecture Notes in Computer Science, 2016

A new reduced state space construction method is presented where in every constructed state, the ... more A new reduced state space construction method is presented where in every constructed state, the set of transitions that are fired is chosen based on the safety property that is being verified. Typical earlier methods only take the property into account in one state of each cycle or in one state of each terminal strong component of the reduced state space. They may fire totally irrelevant transitions in the other states. Where the property is taken into account, typically many or all enabled transitions are fired. This has spoiled attempts to be property-driven in every state. The present study exploits an idea that was published in 2016 with which this can be avoided. Furthermore, most earlier methods classify the transitions to visible and invisible. The new method uses a novel improved concept. An experiment is presented where the new concept provides significant improvement to the reduction results.

Compositional Analysis for Weak Stubborn Sets

Partial order reduction methods rely on expanding a state space only partially, exploring represe... more Partial order reduction methods rely on expanding a state space only partially, exploring representatives of sets of executions of a system. The methods differ at least in three respects:the set of properties that are preserved, the notions of interaction between transitions, and the methods of selecting the representative transitions. We explore an existing but less studied method of weak stubborn sets in the context of process-algebraic parallel composition. The theory of stubborn set methods is based on global condition son executions. In practice, these conditions are guaranteed by using static dependency information in the form of dependency and causality relations. We propose a compositional approach for these conditions. We extract dependency information from the component processes by analysing the components in detail and then define compositional rules for system-level dependency information. We use novel localised conditions of dependency that the weak stubborn set method can make use of. We carry out some experiments in the context of FDR, to explore the relative merits of the method compared to the more well-known version of stubborn sets with and without compositional analysis. We do this for both a deadlock preserving reduction and a version that preserves the failures/divergences semantics of FDR.

Old and New Algorithms for Minimal Coverability Sets

Fundamenta Informaticae, 2014

Many algorithms for computing minimal coverability sets for Petri nets prune futures. That is, if... more Many algorithms for computing minimal coverability sets for Petri nets prune futures. That is, if a newmarking strictly covers an old one, then not just the old marking but also some subset of its successor markings is discarded from search. In this publication, a simpler algorithm that lacks future pruning is presented and proven correct. Its performance is compared with future pruning. It is demonstrated, using examples, that neither approach is systematically better than the other. However, the simple algorithm has some attractive features. It never needs to re-construct pruned parts of the minimal coverability set. It automatically gives most of the advantage of future pruning, if the minimal coverability set is constructed in depth-first or most tokens first order, and if so-called history merging is applied. Some implementation aspects of minimal coverability set construction are also discussed. Some measurements are given to demonstrate the effect of construction order and other implementation aspects.

Partial order reduction for model checking Markov decision processes under unconditional

Fairness assumptions are needed to verify liveness properties of concurrent systems. In this pape... more Fairness assumptions are needed to verify liveness properties of concurrent systems. In this paper we explore the so-called unconditional fairness in Markov decision processes (MDPs), which is a prerequisite for quantitative assumeguarantee reasoning. Unconditional fairness refers to executions where all processes are guaranteed to participate. We prove that realisability of unconditional fairness coincides with the absence of partial deadlocks, i.e., end components where a process suffers from starvation. We propose a weak variant of the stubborn set method to reduce MDPs, while preserving the realisability of unconditional fairness and maximal probabilities of reaching bottom end components under fair schedulers.

Oski Kervinen: An Architecture for Verification with Ex- Tended Labeled Transition Systems

The examiners and topic of the thesis were approved by the Council of the Fac-

Why Confluence is More Powerful than Ample Sets in Probabilistic and Non-Probabilistic Branching Time

Confluence reduction and partial order reduction by means of ample sets are two different techniq... more Confluence reduction and partial order reduction by means of ample sets are two different techniques for state space reduction in both traditional and probabilistic model checking. This presentation provides an extensive comparison between these two methods, answering the long-standing question of how they relate. We show that, while both preserve branching time properties, confluence reduction is strictly more powerful than partial order reduction: every reduction that can be obtained with partial order reduction can also be obtained with confluence reduction, but the converse is not true. A core problem in the comparison is that confluence reduction was defined in an action-based setting, whereas partial order reduction was defined in a state-based setting. We therefore redefine confluence reduction in the state-based setting of Markov decision processes, and discuss a nontrivial proof of its correctness. Additionally, we pinpoint precisely in what way confluence reduction is more...

Teemu Stenhammar Factorization of Binary Polynomials

TEEMU STENHAMMAR: Factorization of binary polynomials Tampere University of Technology Master of ... more