# Publications

Official versions can be found under the DOI reference while extended technical reports are available as an arXiv/.pdf link.

[1] |
Learning Union of Integer Hypercubes with Queries (with applications to
monadic decomposition).
Oliver Markgraf, Daniel Stan, and Anthony Widjaja Lin.
In CAV 2021 (to appear).
We study the problem of learning a finite union of integer (axis-aligned) hypercubes over the d-dimensional integer lattice, i.e., whose edges are parallel to the coordinate axes. This is a natural generalization of the classic problem in the computational learning theory of learning rectangles. We provide a learning algorithm with access to a minimally adequate teacher (i.e. membership and equivalence oracles) that solves this problem in polynomial-time, for any fixed dimension d. Over a non-fixed dimension, the problem subsumes the problem of learning DNF boolean formulas, a central open problem in the field. We have also provided extensions to handle infinite hypercubes in the union, as well as showing how subset queries could improve the performance of the learning algorithm in practice. Our problem has a natural application to the problem of monadic decomposition of quantifier-free integer linear arithmetic formulas, which has been actively studied in recent years. In particular, a finite union of integer hypercubes correspond to a finite disjunction of monadic predicates over integer linear arithmetic (without modulos constraints). Our experiments suggest that our learning algorithms substantially outperform the existing algorithms. |

[2] |
Regular Model Checking Approach to Knowledge Reasoning over Parameterized
Systems.
Daniel Stan and Anthony W. Lin.
In AAMAS 2021.
[ arXiv |
http ]
We present a general framework for modelling and verifying epistemic properties over parameterized multi-agent systems that communicate by truthful public announcements. In our framework, the number of agents or the amount of certain resources are parameterized (i.e. not known a priori), and the corresponding verification problem asks whether a given epistemic property is true regardless of the instantiation of the parameters. For example, in a muddy children puzzle, one could ask whether each child will eventually find out whether (s)he is muddy, regardless of the number of children. Our framework is regular model checking (RMC)-based, wherein synchronous finite-state automata (equivalently, monadic second-order logic over words) are used to specify the systems. We propose an extension of public announcement logic as specification language. Of special interests is the addition of the so-called iterated public announcement operators, which are crucial for reasoning about knowledge in parameterized systems. Although the operators make the model checking problem undecidable, we show that this becomes decidable when an appropriate "disappearance relation" is given. Further, we show how Angluin's L*-algorithm for learning finite automata can be applied to find a disappearance relation, which is guaranteed to terminate if it is regular. We have implemented the algorithm and apply this to such examples as the Muddy Children Puzzle, the Russian Card Problem, and Large Number Challenge. Keywords: automaton learning, epistemic, parameterised, regular model checking, muddy children, public announcement logic |

[3] |
Syntactic Partial Order Compression for Probabilistic Reachability.
Gereon Fox, Daniel Stan, and Holger Hermanns.
In VMCAI 2019.
[ DOI |
.pdf ]
The state space explosion problem is among the largest impediments to the performance of any model checker. Modelling languages for compositional systems contribute to this problem by placing each instruction of an instruction sequence onto a dedicated transition, giving concurrent processes opportunities to interleave after every instruction. Users wishing to avoid the excessive number of interleavings caused by this default can choose to explicitly declare instruction sequences as “atomic”, which however requires careful considerations regarding the impact this might have on the model as well as on the properties that are to be checked. We instead propose a preprocessing technique that automatically identifies instruction sequences that can safely be considered atomic. This is done in the context of concurrent variable-decorated Markov Decision Processes. Our approach is compatible with any off-the-shelf probabilistic model checker. We prove that our transformation preserves maximal reachability probabilities and present case studies to illustrate its usefulness. |

[4] |
Randomized strategies in concurrent games. (Stratégies
randomisées dans les jeux concurrents).
PhD thesis, University of Paris-Saclay, France, 2017.
[ http ]
We study games played on graphs by an arbitrary number of players withnon-zero sum objectives. The players representagents (programs, processes or devices) that can interact to achieve their ownobjectives as much as possible. Solution concepts, as Nash Equilibrium, forsuch optimal plays,need not exist when restricting topure deterministic strategies, even with simple reachability or safetyobjectives. The symmetry induced by deterministic behavioursmotivates the studies where eitherthe players or the environment can use randomization. In the first case, weshow that classical concepts are undecidable with a fixednumber of agents and propose computable approximations. In the second case, we studyrandomization as a reasonable policy for scheduling an arbitrary number of processes. |

[5] |
Stochastic Equilibria under Imprecise Deviations in Terminal-Reward
Concurrent Games.
Patricia Bouyer, Nicolas Markey, and Daniel Stan.
In GandALF 2016.
[ DOI |
.pdf ]
We study the existence of mixed-strategy equilibria in concurrent games played on graphs. While existence is guaranteed with safety objectives for each player, Nash equilibria need not exist when players are given arbitrary terminal-reward objectives, and their existence is undecidable with qualitative reachability objectives (and only three players). However, these results rely on the fact that the players can enforce infinite plays while trying to improve their payoffs. In this paper, we introduce a relaxed notion of equilibria, where deviations are imprecise. We prove that contrary to Nash equilibria, such (stationary) equilibria always exist, and we develop a PSPACE algorithm to compute one. |

[6] |
Reachability in Networks of Register Protocols under Stochastic
Schedulers.
Patricia Bouyer, Nicolas Markey, Mickael Randour, Arnaud Sangnier,
and Daniel Stan.
In ICALP 2016.
[ DOI |
arXiv ]
We study the almost-sure reachability problem in a distributed system obtained as the asynchronous composition of N copies (called processes) of the same automaton (called protocol), that can communicate via a shared register with finite domain. The automaton has two types of transitions: write-transitions update the value of the register, while read-transitions move to a new state depending on the content of the register. Non-determinism is resolved by a stochastic scheduler. Given a protocol, we focus on almost-sure reachability of a target state by one of the processes. The answer to this problem naturally depends on the number N of processes. However, we prove that our setting has a cut-off property: the answer to the almost-sure reachability problem is constant when N is large enough; we then develop an EXPSPACE algorithm deciding whether this constant answer is positive or negative. |

[7] |
Mixed Nash Equilibria in Concurrent Games.
Patricia Bouyer, Nicolas Markey, and Daniel Stan.
In FSTTCS 2014.
[ DOI |
.pdf ]
We study mixed-strategy Nash equilibria in multiplayer deterministic concurrent games played on graphs, with terminal-reward payoffs (that is, absorbing states with a value for each player). We show undecidability of the existence of a constrained Nash equilibrium (the constraint requiring that one player should have maximal payoff), with only three players and 0/1-rewards (i.e., reachability objectives). This has to be compared with the undecidability result by Ummels and Wojtczak for turn-based games which requires 14 players and general rewards. Our proof has various interesting consequences: (i) the undecidability of the existence of a Nash equilibrium with a constraint on the social welfare; (ii) the undecidability of the existence of an (unconstrained) Nash equilibrium in concurrent games with terminal-reward payoffs. |

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