# Sasha Rubin's research while affiliated with The University of Sydney and other places

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## Publications (82)

Strong-cyclic policies" were introduced to formalize trial-and-error strategies and are known to work in Markovian stochastic domains, i.e., they guarantee that the goal is reached with probability 1. We introduce ``best-effort" policies for (not necessarily Markovian) stochastic domains. These generalize strong-cyclic policies by taking advantage...

Do agents know each others’ strategies? In multi-process software construction, each process has access to the processes already constructed; but in typical human-robot interactions, a human may not announce its strategy to the robot (indeed, the human may not even know their own strategy). This question has often been overlooked when modeling and...

We introduce the formula-synthesis problem for Propositional Dynamic Logic with Shuffle (PDL || ). This problem, which generalises the model-checking problem againsts PDL || is the following: given a finite transition system and a regular term-grammar that generates (possibly infinitely many) PDL || formulas, find a formula generated by the grammar...

Recent work has unveiled a theory for reasoning about the decisions made by binary classifiers: a classifier describes a Boolean function, and the reasons behind an instance being classified as positive are the prime-implicants of the function that are satisfied by the instance. One drawback of these works is that they do not explicitly treat scena...

This paper establishes a framework based on logic and automata theory in which to model and automatically verify systems of multiple mobile agents moving in environments with partially-known topologies, i.e., ones which are not completely known at design time. Examples include physical agents designed to be used in many spatial environments and not...

The overall aim of our research is to develop techniques to reason about the equilibrium properties of multi-agent systems. We model multi-agent systems as concurrent games, in which each player is a process that is assumed to act independently and strategically in pursuit of personal preferences. In this article, we study these games in the contex...

We formally introduce and solve the synthesis problem for LTL goals in the case of multiple, even contradicting, assumptions about the environment. Our solution concept is based on ``best-effort strategies'' which are agent plans that, for each of the environment specifications individually, achieve the agent goal against a maximal set of environme...

We study the semantics of knowledge in strategic reasoning. Most existing works either implicitly assume that agents do not know one another’s strategies, or that all strategies are known to all; and some works present inconsistent mixes of both features. We put forward a novel semantics for Strategy Logic with Knowledge that cleanly models whose s...

We study best-effort synthesis under environment assumptions specified in LTL, and show that this problem has exactly the same computational complexity of standard LTL synthesis: 2EXPTIME-complete. We provide optimal algorithms for computing best-effort strategies, both in the case of LTL over infinite traces and LTL over finite traces (i.e., LTLf)...

Planning domains represent what an agent assumes or believes about the environment it acts in. In the presence of nondeterminism, additional temporal assumptions, such as fairness, are often expressed as extra conditions on the domain. Here we consider environment specifications expressed in arbitrary LTL, which generalize many forms of environment...

Recent work has unveiled a theory for reasoning about the decisions made by binary classifiers: a classifier describes a Boolean function, and the reasons behind an instance being classified as positive are the prime-implicants of the function that are satisfied by the instance. One drawback of these works is that they do not explicitly treat scena...

We introduce an extension of Strategy Logic for the imperfect-information setting, called SL ii and study its model-checking problem. As this logic naturally captures multi-player games with imperfect information, this problem is undecidable; but we introduce a syntactical class of “hierarchical instances” for which, intuitively, as one goes down t...

We study alternating automata with qualitative semantics over infinite binary trees: Alternation means that two opposing players construct a decoration of the input tree called a run, and the qualitative semantics says that a run of the automaton is accepting if almost all branches of the run are accepting. In this article, we prove a positive and...

We prove the existence and computability of optimal strategies in weighted limit games, zero-sum infinite-duration games with a B\"uchi-style winning condition requiring to produce infinitely many play prefixes that satisfy a given regular specification. Quality of plays is measured in the maximal weight of infixes between successive play prefixes...

We prove the existence and computability of optimal strategies in weighted limit games, zero-sum infinite-duration games with a B\"uchi-style winning condition requiring to produce infinitely many play prefixes that satisfy a given regular specification. Quality of plays is measured in the maximal weight of infixes between successive play prefixes...

The overall aim of our research is to develop techniques to reason about the equilibrium properties of multi-agent systems. We model multi-agent systems as concurrent games, in which each player is a process that is assumed to act independently and strategically in pursuit of personal preferences. In this article, we study these games in the contex...

We review PLTLf and PLDLf, the pure-past versions of the well-known logics on finite traces LTLf and LDLf , respectively. PLTLf and PLDLf are logics about the past, and so scan the trace backwards from the end towards the beginning. Because of this, we can exploit a foundational result on reverse languages to get an exponential improvement, over LT...

We consider an agent that operates with two models of the environment: one that captures expected behaviors and one that captures additional exceptional behaviors. We study the problem of synthesizing agent strategies that enforce a goal against environments operating as expected while also making a best effort against exceptional environment behav...

We look at program synthesis where the aim is to automatically synthesize a controller that operates on data structures and from which a concrete program can be easily derived. We do not aim at a fully-automatic process or tool that produces a program meeting a given specification of the program’s behaviour. Rather, we aim at the design of a clear...

We address two central notions of fairness in the literature of nondeterministic fully observable domains. The first, which we call stochastic fairness, is classical, and assumes an environment which operates probabilistically using possibly unknown probabilities. The second, which is language-theoretic, assumes that if an action is taken from a gi...

Model checking multi-agent systems, in which agents are distributed and thus may have different observations of the world, against strategic behaviours is known to be a complex problem in a number of settings. There are traditionally two ways of ameliorating this complexity: imposing a hierarchy on the observations of the agents, or restricting age...

We introduce an extension of Strategy Logic for the imperfect-information setting, called SLii, and study its model-checking problem. As this logic naturally captures multi-player games with imperfect information, this problem is undecidable; but we introduce a syntactical class of "hierarchical instances" for which, intuitively, as one goes down t...

We study alternating automata with qualitative semantics over infinite binary trees: alternation means that two opposing players construct a decoration of the input tree called a run, and the qualitative semantics says that a run of the automaton is accepting if almost all branches of the run are accepting. In this paper we prove a positive and a n...

We address two central notions of fairness in the literature of planning on nondeterministic fully observable domains. The first, which we call stochastic fairness, is classical, and assumes an environment which operates probabilistically using possibly unknown probabilities. The second, which is language-theoretic, assumes that if an action is tak...

We introduce a logic to reason about strategic abilities in finite games under imperfect information. We interpret Alternating-time Temporal Logic on interpreted systems with final states, where agents only have partial observability of the system’s global state. We consider the model checking problem in this setting. We prove that the complexity r...

We study the characterization and computation of general policies for families of problems that share a structure characterized by a common reduction into a single abstract problem. Policies $\mu$ that solve the abstract problem P have been shown to solve all problems Q that reduce to P provided that $\mu$ terminates in Q. In this work, we shed lig...

This paper introduces Graded Computation Tree Logic with finite path semantics (GCTLf⁎, for short), a variant of Computation Tree Logic CTL⁎, in which path quantifiers are interpreted over finite paths and can count the number of such paths. State formulas of GCTLf⁎ are interpreted over Kripke structures. The syntax of GCTLf⁎ has path quantifiers o...

We introduce Probabilistic Strategy Logic, an extension of Strategy Logic for stochastic systems. The logic has probabilistic terms that allow it to express many standard solution concepts, such as Nash equilibria in randomised strategies, as well as constraints on probabilities, such as independence. We study the model-checking problem for agents...

Planning domains represent what an agent assumes or believes about the environment it acts in. In the presence of nondeterminism, additional temporal assumptions, such as fairness, are often expressed as extra conditions on the domain. Here we consider environment specifications expressed in arbitrary LTL, which generalize many forms of environment...

We study dynamic changes of agents’ observational power in logics of knowledge and time. We consider CTLK*, the extension of CTL* with knowledge operators, and enrich it with a new operator that models a change in an agent’s way of observing the system. We extend the classic semantics of knowledge for agents with perfect recall to account for chang...

We consider an extension of monadic second-order logic, interpreted over the infinite binary tree, by the qualitative path-measure quantifier. This quantifier says that the set of infinite paths in the tree satisfying a formula has Lebesgue-measure one. We prove that this logic is undecidable. To do this we prove that the emptiness problem of quali...

In this paper we introduce Graded Computation Tree Logic over finite paths (GCTL_f* , for short), a variant of Computation Tree Logic CTL* , in which path quantifiers are interpreted over finite paths and can count the number of such paths. State formulas of GCTL_f* are interpreted over Kripke structures with a designated set of states, which we ca...

Strategy Logic (SL) is a logical formalism for strategic reasoning in multi-agent systems. Its main feature is that it has variables for strategies that are associated to specific agents using a binding operator. In this paper we introduce Graded Strategy Logic (GradedSL), an extension of SL by graded quantifiers over tuples of strategy variables,...

Program synthesis constructs programs from specifications in an automated way. Strategy Logic (SL) is a powerful and versatile specification language whose goal is to give theoretical foundations for program synthesis in a multi-agent setting. One limitation of Strategy Logic is that it is purely qualitative. For instance it cannot specify quantita...

In Reasoning about Action and Planning, one synthesizes the agent plan by taking advantage of the assumption on how the environment works (that is, one exploits the environment's effects, its fairness, its trajectory constraints). In this paper we study this form of synthesis in detail. We consider assumptions as constraints on the possible strateg...

We study planning for LTLf and LDLf temporally extended goals in nondeterministic fully observable domains (FOND). We consider both strong and strong cyclic plans, and develop foundational automata-based techniques to deal with both cases. Using these techniques we provide the computational characterization of both problems, separating the complexi...

We develop a logic-based technique to analyse finite interactions in multi-agent systems. We introduce a semantics for Alternating-time Temporal Logic (for both perfect and imperfect recall) and its branching-time fragments in which paths are finite instead of infinite. We study validities of these logics and present optimal algorithms for their mo...

Parameterized model checking is the problem of deciding if a given formula holds irrespective of the number of participating processes. A standard approach for solving the parameterized model checking problem is to reduce it to model checking finitely many finite-state systems. This work considers the theoretical power and limitations of this techn...

We study dynamic changes of agents' observational power in logics of knowledge and time. We consider CTL*K, the extension of CTL* with knowledge operators, and enrich it with a new operator that models a change in an agent's way of observing the system. We extend the classic semantics of knowledge for perfect-recall agents to account for changes of...

Graded path modalities count the number of paths satisfying a property, and generalize the existential (E) and universal (A) path modalities of [Figure presented]. The resulting logic is denoted G [Figure presented], and is a powerful logic since (as we show) it is equivalent, over trees, to monadic path logic. We establish the complexity of the sa...

We study concurrent games with finite-memory strategies where players are given a Buchi and a mean-payoff objective, which are related by a lexicographic order: a player first prefers to satisfy its Buchi objective, and then prefers to minimise costs, which are given by a mean-payoff function. In particular, we show that deciding the existence of a...

We study a class of synchronous, perfect-recall multi-agent systemswith imperfect information and broadcasting (i.e., fully observableactions). We define an epistemic extension of strategy logic withincomplete information and the assumption of uniform and coherentstrategies. In this setting, we prove that the model checking problem,and thus rationa...

We study the characterization and computation of general policies for families of problems that share a structure characterized by a common reduction into a single abstract problem. Policies mu that solve the abstract problem P have been shown to solve all problems Q that reduce to P provided that mu terminates in Q. In this work, we shed light on...

We introduce an extension of Strategy logic for the imperfect-information setting, called SL ii , and study its model-checking problem. As this logic naturally captures multi-player games with imperfect information, the problem turns out to be undecidable. We introduce a syntactical class of " hierarchical instances " for which, intuitively, as one...

We introduce an extension of Strategy logic for the imperfect-information setting, called SL ii , and study its model-checking problem. As this logic naturally captures multi-player games with imperfect information, the problem turns out to be undecidable. We introduce a syntactical class of " hierarchical instances " for which, intuitively, as one...

We analyse the verification problem for synchronous, perfect recall multi-agent systems with imperfect information against a specification language that includes strategic as well as epistemic operators. While the general problem is known to be undecidable we show that if the agents' actions are public then verification is decidable, and we establi...

First-cycle games (FCG) are played on a finite graph by two players who push a token along the edges until a vertex is repeated, and a simple cycle is formed. The winner is determined by some fixed property Y of the sequence of labels of the edges (or nodes) forming this cycle. These games are intimately connected with classic infinite-duration gam...

We consider the model checking problem of infinite state systems given in the form of parameterized discrete timed networks with multiple clocks. We show that this problem is decidable with respect to specifications given by B- or S-automata. Such specifications are very expressive (they strictly subsume omega-regular specifications), and easily ex...

In this work we build on these models to look at social influence from a strategic perspective. We do so by introducing a new class of games, called games of influence. Specifically, a game of influence is an infinite repeated game with incomplete information in which, at each stage of interaction, an agent can make her opinions visible (public) or...

Strategy Logic (SL) is a logical formalism for strategic reasoning in multi-agent systems. Its main feature is that it has variables for strategies that are associated to specific agents with a binding operator. We introduce Graded Strategy Logic (GradedSL), an extension of SL by graded quantifiers over tuples of strategy variables, i.e., "there ex...

We study the model checking problem of parameterised systems with an arbitrary number of processes, on arbitrary network-graphs, communicating using multiple multi-valued tokens, and specifications from indexed-branching temporal logic. We prove a composition theorem, in the spirit of Feferman-Vaught [21] and Shelah [31], and a finiteness theorem,...

We present a framework for modeling and analysing multiple mobile agents on grid-environments such as finite mazes and labyrinths. Agents are modeled as automata, and the grid-environments are parameterised by their size and the relative positions of the obstacles. We study the verification problem, i.e., whether given agents complete a given task...

In temporal logics, the operator F expresses that at some time in the future something happens, e.g., a request is eventually granted. Unfortunately, there is no bound on the time until the eventuality is satisfied which in many cases does not correspond to the intuitive meaning system designers have, namely, that F abstracts the idea that there is...

Graded path modalities count the number of paths satisfying a property, and generalize the existential (E) and universal (A) path modalities of CTL*. The resulting logic is denoted GCTL*, and is a very powerful logic since
(as we show) it is equivalent, over trees, to monadic path logic. We settle the complexity of the satisfiability problem of GCT...

We study foundational problems regarding the expressive power of parameterised systems. These (infinite-state) systems are composed of arbitrarily many finite-state processes that synchronise using a given communication primitive, i.e., broadcast, asynchronous rendezvous, broadcast with message loss, pairwise rendezvous, or disjunctive guards. With...

This paper establishes a framework based on logic and automata theory in which to model and automatically verify that multiple mobile robots, with sensing abilities, moving asynchronously, correctly perform their tasks. The motivation is from practical scenarios in which the environment is not completely know to the robots, e.g., physical robots ex...

This paper establishes a framework based on logic and automata theory in which to model and automatically verify that multiple mobile robots, with sensing abilities, moving asynchronously, correctly perform their tasks. The motivation is from practical scenarios in which the environment is not completely know to the robots, e.g., physical robots ex...

We consider the problem of planning paths of multiple agents in a dynamic but predictable environment. Typical scenarios are evacuation, reconfiguration, and containment. We present a novel representation of abstract path-planning problems in which the stationary environment is explicitly coded as a graph while the dynamic environment is treated as...

While the classic model checking problem is to decide whether a finite system satisfies a specification, the goal of parameterized model checking is to decide, given finite systems M(n) parameterized by n ∈ N, whether, for all n ∈ N, the system M(n) satisfies a specification. In this book we consider the important case of M(n) being a concurrent sy...

We consider the model checking problem of infinite state systems given in the form of parameterized discrete timed networks with multiple clocks. We show that this problem is decidable with respect to specifications given by B-or S-automata. Such specifications are very expressive (they strictly subsume ω-regular specifications), and easily express...

In a token-passing system (TPS), processes communicate by passing a token to their neighbors in the connectivity graph. We will define the passing of a token (with or without a value) as a special case of pairwise-rendezvous synchronization. In contrast to most of the other classes of systems in this survey, TPSs have been analyzed on complex conne...

In this book we focused on established decidability results in parameterized model checking. As this topic is a lively field, we cannot make any claim about completeness of our survey. In particular, we left out the large body of research that includes invariant-based techniques, regular model checking, symbolic methods, techniques for counter auto...

In ad hoc networks, processes communicate via broadcasts. Recall from Section 2.2.1 that in transitions synchronized by broadcast, all immediate neighbors of a sender simultaneously take a transition with the sender, and the sender is not blocked if there are no recipients that are ready to synchronize. This chapter summarizes the results by Delzan...

This chapter considers systems of processes communicating via pairwise rendezvous, asynchronous rendezvous, broadcast, and combinations of these (as defined in Section 2.2.1). We have already seen in Examples 2.1 and 2.2 how pairwise-rendezvous models standard synchronization between exactly two processes. As discussed by Delzanno et al. [2002], as...

The purpose of this section is to briefly summarize the typical principles and ingredients in proofs of (un)decidability for the PMC problem.

In this chapter we present definitions of parameterized systems, parameterized specifications and the parameterized model-checking problem. The rationale behind our definitions is to make explicit many of the commonalities of the different system models and results found in the literature.

In this chapter we extend the computational model of Section 2.2 with a new means of coordination. Until now processes coordinated with synchronized transitions and we have used transition labels to express which transitions should be taken together. In this chapter, we introduce guards, which are conditions on the global state and determine whethe...

The results covered in this book show that the parameterized model checking problem is only decidable for rather restricted classes of system models and specifications. In addition to that, parameterized model checking is computationally hard even in cases where it is decidable. As a result, there are only few software tools that implement decision...

Graded path modalities count the number of paths satisfying a property, and
generalize the existential (E) and universal (A) path modalities of CTL*. The
resulting logic is called GCTL*. We settle the complexity of satisfiability of
GCTL*, i.e., 2ExpTime-Complete, and the complexity of the model checking
problem for GCTL*, i.e., PSpace-Complete. Th...

A standard technique for solving the parameterized model checking problem is to reduce it to the classic model checking problem of finitely many finite-state systems. This work considers some of the theoretical power and limitations of this technique. We focus on concurrent systems in which processes communicate via pairwise rendezvous, as well as...

First cycle games (FCG) are played on a finite graph by two players who push a token along the edges until a vertex is repeated, and a simple cycle is formed. The winner is determined by some fixed property Y of the sequence of labels of the edges (or nodes) forming this cycle. These games are traditionally of interest because of their connection w...

We revisit the parameterized model checking problem for token-passing systems and specifications in indexed CTL
∗ \X. Emerson and Namjoshi (1995, 2003) have shown that parameterized model checking of indexed CTL
∗ \X in uni-directional token rings can be reduced to checking rings up to some cutoff size. Clarke et al. (2004) have shown a similar res...