Editors: Nicolas Markey and Patricia Bouyer

Authors, Title and AbstractPaperTalk

ABSTRACT. Two-player games on graphs are widely studied in formal methods as they model the interaction between a system and its environment. The game is played by moving a token throughout a graph to produce an infinite path. There are several common modes to determine how the players move the token through the graph; e.g., in turn-based games the players alternate turns in moving the token. We study the {\em bidding} mode of moving the token, which, to the best of our knowledge, has never been studied in infinite-duration games. Both players have separate {\em budgets}, which sum up to $1$. In each turn, a bidding takes place. Both players submit bids simultaneously, and a bid is legal if it does not exceed the available budget. The winner of the bidding pays his bid to the other player and moves the token. Reachability bidding games, called {\em Richman games}, have been studied in \cite{LLPU96,LLPSU99}. There, a central question is the existence and computation of {\em threshold} budgets; namely, a value $t \in [0,1]$ such that if \PO's budget exceeds $t$, he can win the game, and if \PT's budget exceeds $1-t$, he can win the game. We focus on parity and mean-payoff games. We show the existence of threshold budgets and show that the complexity of finding them coincides with the $NP \cap coNP$ complexity of reachability bidding games. The solution for mean-payoff consists of our most technically challenging contribution, where we construct optimal strategies for the players while extending and generalizing the probabilistic connection that was known for reachability bidding games.

Jul 08 14:00

ABSTRACT. Admissible strategies, i.e. those that are not dominated by any other strategy, are a typical rationality notion in game theory. In many classes of games this is justified by results showing that any strategy is admissible or dominated by an admissible strategy. However, in games played on finite graphs with quantitative objectives (as used for reactive synthesis), this is not the case.

We consider increasing chains of strategies instead to recover a satisfactory rationality notion based on dominance in such games. We start with some order-theoretic considerations establishing sufficient criteria for this to work. We then turn our attention to generalised safety/reachability games as a particular application. We propose the notion of maximal uniform chain as the desired dominance-based rationality concept in these games. Decidability of some fundamental questions about uniform chains is established.

Jul 07 17:20

ABSTRACT. We analyse the verification problem for synchronous, perfect recall multi-agent systems with imperfect information against a specification language that includes strategic and epistemic operators. While the verification problem is undecidable, we show that if the agents' actions are public, then verification is 2 EXPTIME -complete. To illustrate the formal framework we consider two epistemic and strategic puzzles with imperfect information and public actions: the muddy children puzzle and the classic game of battleships. This paper has been accepted for publication at AAMAS2017.

Jul 07 16:00

ABSTRACT. 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, the problem turns out to be undecidable. We introduce a syntactical class of "hierarchical instances" for which, intuitively, as one goes down the syntactic tree of the formula, strategy quantifications are concerned with finer observations of the model. We prove that model-checking SLii restricted to hierarchical instances is decidable. This result, because it allows for complex patterns of existential and universal quantification on strategies, greatly generalises previous ones, such as decidability of multi-player games with imperfect information and hierarchical observations, and decidability of distributed synthesis for hierarchical systems.

Jul 08 16:40

ABSTRACT. In this paper we provide a broad investigation of the symbolic approach for solving Parity Games. Specifically, we implement in a fresh tool, called SymPGSolver, four symbolic algorithms to solve Parity Games and compare their performances to the corresponding explicit versions for different classes of games. By means of benchmarks, we show that for random games, even for constrained random games, explicit algorithms actually perform better than symbolic algorithms. The situation changes, however, for structured games, where symbolic algorithms seem to have the advantage. This suggests that when evaluating algorithms for parity-game solving, it would be useful to have real benchmarks and not only random benchmarks, as the common practice has been.

Jul 08 14:40

ABSTRACT. Program synthesis automatically constructs programs from specifications. Strategy Logic is a powerful 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 quantitative properties of executions such as "every request is quickly granted", or quantitative properties of trees such as "most executions of the system terminate". In this work, we extend Strategy Logic to include quantitative aspects in a way that can express bounds on "how quickly" and "how many". We define Prompt Strategy Logic, which encompasses Prompt LTL (itself an extension of LTL with a prompt eventuality temporal operator), and we define Bounded-Outcome Strategy Logic which has a bounded quantifier on paths. We supply a general technique, based on the study of automata with counters, that solves the model-checking problems for both these logics.

Jul 08 16:00

ABSTRACT. Strategy Logic (SL) is a very expressive temporal logic for specifying and verifying properties of multi-agent systems: in SL, one can quantify over strategies, assign them to agents, and express LTL properties of the resulting plays. Such a powerful framework has two drawbacks: first, model checking SL has non-elementary complexity; second, the exact semantics of SL is rather intricate, and may not correspond to what is expected. In this paper, we focus on strategy dependences in SL, by tracking how existentially-quantified strategies in a formula may (or may not) depend on other strategies selected in the formula, revisiting the approach of [Mogavero et al., Reasoning about strategies: On the model-checking problem, 2014]. We explain why elementary dependences, as defined by Mogavero et al., do not exactly capture the intended concept of behavioral strategies. We address this discrepancy by introducing timeline dependences, and exhibit a large fragment of SL for which model checking can be performed in 2EXPTIME under this new semantics.

Jul 08 17:20

ABSTRACT. We introduce open games as a compositional foundation of economic game theory. A compositional approach potentially allows methods of game theory and theoretical computer science to be applied to large-scale economic models for which standard economic tools are not practical. An open game represents a game played relative to an arbitrary environment and to this end we introduce the concept of coutility, which is the utility generated by an open game and returned to its environment. Open games are the morphisms of a symmetric monoidal category and can therefore be composed by categorical composition into sequential move games and by monoidal products into simultaneous move games. Open games can be represented by string diagrams which provide an intuitive but formal visualisation of the information flows. We show that a variety of games can be faithfully represented as open games in the sense of having the same Nash equilibria and off-equilibrium best responses.

Jul 07 14:00

ABSTRACT. Game theory provides a well-established framework for the analysis of multi-agent systems. Typically, the analysis of a multi-agent system involves computing the set of equilibria in the associated multi-player game representing the behaviour of the system. As systems grow larger, it becomes increasingly harder to find equilibria in the game -- which represent the rationally stable behaviours of the multi-agent system (the solutions of the game). To address this issue, in this paper, we study the concept of local equilibria, which are defined with respect to (maximal) stable coalitions of agents for which an equilibrium can de found. We focus on the solutions given by the Nash equilibria of Boolean games and iterated Boolean games, two logic-based models for multi-agent systems, in which the players' goals are given by formulae of propositional logic and LTL, respectively.

Jul 07 16:40

ABSTRACT. Consider concurrent, infinite duration, two-player win/lose games played on graphs. If the winning condition satisfies some simple requirement, the existence of Player 1 winning (finite-memory) strategies is equivalent to the existence of winning (finite-memory) strategies in finitely many derived one-player games. Several classical winning conditions satisfy this simple requirement.

Under an additional requirement on the winning condition, the non-existence of Player 1 winning strategies from all vertices is equivalent to the existence of Player 2 stochastic strategies winning almost surely from all vertices. Only few classical winning conditions satisfy this additional requirement, but a fairness variant of o

Jul 07 14:40