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Arena-Independent Finite Memory
Patricia Bouyer, Stéphane Le Roux, Youssouf Oualhadj, Mickael Randour, Pierre Vandenhove
To cite this version:
Patricia Bouyer, Stéphane Le Roux, Youssouf Oualhadj, Mickael Randour, Pierre Vanden- hove. Games Where You Can Play Optimally with Arena-Independent Finite Memory. 31st International Conference on Concurrency Theory (CONCUR’20), Sep 2020, Vienna, Austria.
�10.4230/LIPIcs.CONCUR.2020.18�. �hal-02917552�
Arena-Independent Finite Memory
2
Patricia Bouyer
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LSV – CNRS & ENS Paris-Saclay, Université Paris-Saclay, France
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Stéphane Le Roux
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LSV – CNRS & ENS Paris-Saclay, Université Paris-Saclay, France
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Youssouf Oualhadj
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LACL – UPEC, France
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Mickael Randour
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F.R.S.-FNRS & UMONS – Université de Mons, Belgium
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Pierre Vandenhove
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F.R.S.-FNRS & UMONS – Université de Mons, Belgium
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LSV – CNRS & ENS Paris-Saclay, Université Paris-Saclay, France
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Abstract
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For decades, two-player (antagonistic) games on graphs have been a framework of choice for many
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important problems in theoretical computer science. A notorious one is controller synthesis, which
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can be rephrased through the game-theoretic metaphor as the quest for a winning strategy of the
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system in a game against its antagonistic environment. Depending on the specification,optimal
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strategies might be simple or quite complex, for example having to use (possibly infinite) memory.
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Hence, research strives to understand which settings allow for simple strategies.
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In 2005, Gimbert and Zielonka [27] provided a complete characterization of preference relations
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(a formal framework to model specifications and game objectives) that admitmemorylessoptimal
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strategies for both players. In the last fifteen years however, practical applications have driven the
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community toward games with complex or multiple objectives, where memory — finite or infinite —
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is almost always required. Despite much effort, the exact frontiers of the class of preference relations
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that admitfinite-memoryoptimal strategies still elude us.
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In this work, we establish a complete characterization of preference relations that admit optimal
27
strategies usingarena-independent finite memory, generalizing the work of Gimbert and Zielonka to
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the finite-memory case. We also prove an equivalent to their celebrated corollary of great practical
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interest: if both players have optimal (arena-independent-)finite-memory strategies in all one-player
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games, then it is also the case in all two-player games. Finally, we pinpoint the boundaries of our
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results with regard to the literature: our work completely covers the case of arena-independent
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memory (e.g., multiple parity objectives, lower- and upper-bounded energy objectives), and paves
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the way to the arena-dependent case (e.g., multiple lower-bounded energy objectives).
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2012 ACM Subject Classification Theory of computation→Formal languages and automata theory
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Keywords and phrases two-player games on graphs, finite-memory determinacy, optimal strategies
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Digital Object Identifier 10.4230/LIPIcs.CONCUR.2020.18
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Related Version Full paper on arXiv: https://arxiv.org/abs/2001.03894
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Funding Research supported by F.R.S.-FNRS under Grant n◦F.4520.18 (ManySynth), F.R.S.-FNRS
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mobility funding for scientific missions (Y. Oualhadj in UMONS, 2018), and ENS Paris-Saclay
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visiting professorship (M. Randour, 2019). Mickael Randour is an F.R.S.-FNRS Research Associate
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and Pierre Vandenhove is an F.R.S.-FNRS Research Fellow.
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Acknowledgements We extend our warmest thanks to Mathieu Sassolas, for inspiring discussions
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that were essential in starting this work.
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© Patricia Bouyer, Stéphane Le Roux, Youssouf Oualhadj, Mickael Randour, and Pierre Vandenhove;
licensed under Creative Commons License CC-BY
1 Introduction
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Controller synthesis through the game-theoretic metaphor. Two-player games on (finite)
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graphs are studied extensively, in particular for their application to controller synthesis for
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reactive systems (see, e.g., [29,37,7,2]). The seminal model is antagonistic(i.e.,zero-sumif
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one chooses a quantitative view): player 1 (P1) is seen as the system to control, player 2 (P2)
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as its antagonistic environment, and the game models their interaction. Each vertex of the
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game graph (calledarena) models astate of the system and belongs to one of the players.
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Players take turns moving a pebble from state to state according to the edges, each player
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choosing the destination whenever the pebble is on one of his states. These choices are made
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according to thestrategy of the player, which, in general, might use memory (bounded or
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not) of the past moves to prescribe the next action.
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The resulting infinite sequence of states, called play, represents the execution of the
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system. The objective ofP1 is to enforce a givenspecification, often encoded as awinning
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condition(i.e., a set of winning plays) or as apayoff functionto maximize (i.e., a quantitative
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performance to optimize). This paradigm focuses on the worst-case performance of the
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system, henceP2’s goal is to preventP1from achieving his objective.
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The goal ofsynthesis is thus to decide ifP1 has a winning strategy, i.e., one ensuring
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a given winning condition or guaranteeing a given payoff threshold, against all possible
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strategies ofP2, and to build such a strategy efficiently if it exists.
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Winning strategies are formal blueprints for controllers to implement in applications.
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Therefore, their complexity is of tremendous importance: the simpler the strategy, the easier
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and cheaper it will be to build the controller and maintain it. This explains why a lot of
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research effort is constantly put in identifying the exact complexity (in terms of memory and/or
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randomness) of strategies needed to playoptimally (i.e., to the best of the player’s ability)
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for each specific class of games and objectives (e.g., [27, 17,14,42,22,10,1,4, 41,5,11]).
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Alongside the practical interest lies the theoretical puzzle: understanding the underlying
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mechanisms and implicit properties of games that lead to “simple” strategies being sufficient.
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Given the numerous connections between games and various branches of mathematics and
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computer science, this fundamental question has interest in its own right.
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Preference relations. There are two prominent ways to formalize a game objective in the
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literature. The first one, dubbedquantitativeand inspired by games in economics, is to use
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payoff functionsmapping plays to numerical values, and to seeP1 as a maximizer player.
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This is for example the case of mean-payoff games [19]. The second one, calledqualitative, is
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to define aset of winning plays — calledwinning condition — induced by some property, as
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in, e.g., parity games [20,43]. The two formalisms are strongly linked: the classical decision
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problem for quantitative games is to fix a payoff threshold and ask ifP1 has a strategy to
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guarantee it, transforming the problem into a qualitative one (where the winning plays are
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all those with a payoff at least equal to the threshold). To define payoff functions or winning
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conditions, one often uses weights, priorities, colors, etc, on states or edges of the arena.
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In this work, we walk in the footsteps of Gimbert and Zielonka [27]: we associate acolor
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to each edge of our arenas, and we adopt the abstract formalism ofpreference relations over
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infinite sequences of colors (induced by plays). This general formalism permits to encode
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virtually all classical game objectives, both qualitative and quantitative, and lets us reason
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in a well-founded framework under minimal assumptions.
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Memoryless optimal strategies. Remarkably, several canonical classes of games that have
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been around for decades and proved their usefulness over and over — e.g., mean-payoff [19],
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parity [20,43], or energy games [12] — share a desirable property: they all admitmemoryless
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optimal strategies for both players. That is, for every strategy σi ofPi, there is a strategy
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σMLi which isat least as good (i.e., wins wheneverσiwins or ensures at least the same payoff)
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and that uses no memory at all. Such a memoryless strategy always picks the same edge
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when in the same state, regardless of what happened before in the game.
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Memoryless strategies are the simplest kind of strategies one can use. Therefore, it
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is quite interesting that they suffice for objectives as rich as the ones we just discussed.
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Following this observation, a lot of effort has been put in understanding which games
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admit memoryless optimal strategies, and in identifying the exact frontiers ofmemoryless
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determinacy. Let us mention, non-exhaustively, works by Gimbert and Zielonka [26, 27]
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(culminating in a complete characterization), Aminof and Rubin [1] (through the prism of
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first-cycle games), and Kopczyński [33] (half-positional determinacy). All these advances
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were built by identifying the common underlying mechanisms in ad hoc proofs for specific
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classes of games, and generalizing them to wide classes (e.g., the first-cycle games of [1] are
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inspired by the seminal paper of Ehrenfeucht and Mycielski on mean-payoff games [19]).
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Gimbert and Zielonka’s approach. Arguably, the most important result in this direction is
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thecomplete characterizationof preference relations admitting memoryless optimal strategies,
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established in [27], fifteen years ago. By complete characterization, we mean sufficientand
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necessary conditions on the preference relations.
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It can be stated as follows: a preference relation admits memoryless optimal strategies
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for both players on all arenas if and only if the relation (used byP1) and its inverse (used
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by P2) aremonotone andselective. These concepts will be defined formally in Section 3,
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but let us give an intuition here. Roughly, a preference relation is monotone if it is stable
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underprefix addition: given two sequences of colors such that one isstrictly preferred to the
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other, it is impossible to reverse this order of preference by adding the same prefix to both
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sequences. Selectivity is similarly defined with regard tocycle mixing: if a preference relation
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is selective, then, starting from two sequences of colors, it is impossible to create a third one
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by mixing the first two in such a way that the third one is strictly preferred to the first two.
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These elegant notions coincide with the natural intuition that memoryless strategies suffice if
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there is no interest in behaving differently in a state depending on what happened before.
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In addition to this complete characterization, Gimbert and Zielonka proved another great
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result, of high interest in practice [27, Corollary 7]: as a by-product of their approach, they
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obtain that if memoryless strategies suffice in all one-player games ofP1 and all one-player
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games of P2, they also suffice in all two-player games. Such a lifting corollary provides a
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neat and easy way to prove that a preference relation admits memoryless optimal strategies
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without proving monotony and selectivity at all: proving it in the two one-player subcases,
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which is generally much easier as it boils down to graph reasoning, and then lifting the result
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to the general two-player case through the corollary.
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The rise of memory. The need to model complex specifications has shifted research toward
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games where multiple (quantitative and qualitative) objectives co-exist and interact, requiring
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the analysis of interplay andtrade-offs between several objectives. Hence, a lot of effort
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is put in studying games where objectives are actually conjunctions of objectives, or even
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richer Boolean combinations. See for example [16] for combinations of parity, [13,17,32] for
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combinations of energy and parity, [42] for combinations of mean-payoff, [5,4] for combinations
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of energy and average-energy, [11] for combinations of energy and mean-payoff, [14] for
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combinations of total-payoff, or [14,10,8] for combinations of window objectives.
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s2 s1
1
1
−1
−1
Figure 1P1 (circle) needs infinite memory to win.
When considering such rich objectives,memoryless strategies usually do not suffice, and
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one has to use an amount of memory that can quickly become an obstacle to implementation
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(e.g., exponential memory) or that can prevent it completely (infinite memory). Establishing
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precise memory bounds for such general combinations of objectives is tricky and sometimes
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counterintuitive. For example, while energy games and mean-payoff games are inter-reducible
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in the single-objective setting, exponential-memory strategies are both sufficient and necessary
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for conjunctions of energy objectives [17,32] whileinfinite-memory strategies are required
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for conjunctions of mean-payoff ones [42].
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A natural question arises: which preference relations do admit finite-memory optimal
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strategies? Surprisingly, whether an equivalent to Gimbert and Zielonka’s characterization
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could be obtained for finite memory or not has remained an open question up to now. It is
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worth noticing that such an equivalent could be of tremendous help in practice, especially if
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alifting corollary also holds: see for example [5,4,11], where proving that finite-memory
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strategies suffice in one-player games was fairly easy, in contrast to the high complexity of
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the two-player case — a lifting corollary could grant the two-player case for free!
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Having said that, one has to hope that the following corollary can be established: “if
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finite-memory strategies suffice in all one-player games ofP1and all one-player games of P2,
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they also suffice in all two-player games.” Unfortunately, this hope is but a delusion.
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Lifting corollary: a counterexample. Consider games where the colors are integers, and
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the objective ofP1 is to create a play such that (a) the running sum of weights grows up to
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infinity (e.g., consider its lim inf to define it properly),or (b) this running sum of weights
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takes value zero infinitely often. As this defines a qualitative objective, the corresponding
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preference relation induces only two equivalence classes: winning and losing plays. The
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inverse relation, used byP2, is trivial to obtain. It is fairly easy to prove thatP1 always
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has finite-memory optimal strategies in his one-player games (i.e., games whereP2 has no
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choice), and so doesP2 in his one-player games.
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Now, consider the very simple two-player game depicted in Figure1. PlayerP1(circle)
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has aninfinite-memory strategy to win: keeping track of the running sum of weights (which
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is unbounded, hence the need for infinite memory) and looping ins1up to the point where
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this sum hits zero, then going tos2. This strategy ensures victory because eitherP2always
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goes back tos1, in which case (b) is satisfied; orP2 eventually loops forever ons2, in which
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case (a) is satisfied. It remains to argue thatP1has nofinite-memory winning strategy in
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this game. This can be done using a standard argument: whatever the amount of memory
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used byP1,P2 may loop ins2 long enough as to exceed the bound up to whichP1can track
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the sum accurately; thus doomingP1 to fail to reset the sum to zero ins1 infinitely often.
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This modest example proves thatGimbert and Zielonka’s approach cannot work in full
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generality in the finite-memory case, and for good reasons. Informally, in this case, the
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corollary breaks down because of (the absence of some sort of) monotony. In the case of
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memorylessstrategies, as in [27],P1is already doomed in one-player games in the absence of
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monotony: two prefixes to distinguish — in order to play optimally — can be hardcoded as
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different paths leading to the same state in a game arena, as if they were chosen by P2 in a
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two-player game. In the case offinite-memorystrategies, however, the situation is different.
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In one-player games, the number of such paths that can be hardcoded in an arena is always
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bounded, hence finite memory might suffice to react, i.e., to keep track of which prefix is the
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current one and how to behave accordingly. However, in two-player games,P2 might create
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an infinitenumber of prefixes to distinguish (using a cycle), thus requiringP1 to use infinite
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memory to be able to do so. This is exactly what happens in the example above: in any
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one-player game, the largest sum thatP1 has to track is bounded, whereasP2 can make this
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sum as large as he wants in two-player games.
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Our approach. We generalize Gimbert and Zielonka’s results — characterizationand lifting
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corollary — to the case of arena-independent finite memory. That is, we encompass all
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situations where the memory needed by the two players issolely dependent on the preference
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relation(e.g., colors, dimensions of weight vectors), and noton the game arena (i.e., number
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of edges/states). Let us take some examples.
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All memoryless-determined relations — studied in [27] — use arena-independent memory:
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the memory required, none, is the same for all arenas.
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Combinations of parity objectives use arena-independent memory [16]: the memory only
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depends on the number of objectives and the number of priorities — both parameters of
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the preference relation, not on the size of the arena.
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Lower- and upper-bounded energy objectives also use arena-independent memory (see,
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e.g., [3,5, 4]): the memory only depends on the bounds and the weights — parameters
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of the preference relation, not on the size of the arena.
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On the contrary, combinations of lower-bounded energy objectives (with no upper bound)
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requirearena-dependent memory [17,32]: it depends on the size of the arena in addition
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to the weights used in it. Such a setting falls outside the scope of our results.
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This informal concept of arena-independent memory is transparent in our work: in all
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our results, we usememory skeletons — essentially Mealy machines without a next-action
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function (Section2) — that suffice forall arenas, and that are at the basis of the strategies
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we build. A quick look at our main concepts (Section3) and results (Section4) suffices to
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grasp the formalism behind this intuition.
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This restriction to arena-independent memory is natural given the counterexample to a
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general approach presented above. It is also important to note that it is not as restrictive as
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it may seem, as hinted by the examples above: we arenot restricted to constant memory
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but to memory only depending on theparameters of the preference relation (or equivalently,
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objective), and not of the arena. This framework thus already encompasses many objectives
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from the literature — e.g., [19, 20, 43, 12, 5, 21, 16, 10, 14, 3, 5, 4], as well as possible
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extensions. More details in AppendixA.
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The arena-independent case, which we solve here, is an exact equivalent to Gimbert and
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Zielonka’s results in the finite-memory case: the memoryless case is de facto arena-independent.
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Therefore, this paper strictly generalizes [27] by allowing to study any arena-independent
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memory skeleton instead of the unique trivial one corresponding to memoryless strategies.
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Outline of our contributions. Informally, our characterization can be stated as follows:
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given a preference relation and a memory skeleton M, both players have optimal finite-
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memory strategies based on skeletonMin all games if and only if the relation and its inverse
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areM-monotone andM-selective.
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These last two concepts are keys to our approach. Intuitively, they correspond to Gimbert
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and Zielonka’s monotony and selectivity,modulo a memory skeleton. Recall that monotony
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and selectivity are related to stability of the preference relation with regard to prefix addition
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and cycle mixing, respectively. Our more general concepts ofM-monotony andM-selectivity
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serve the same purpose, but they only compare sequences of colors that are deemed equivalent
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by the memory skeleton. For the sake of illustration, take selectivity: it implies that one has
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no interest in mixing different cycles of the game arena. For its generalization, the memory
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skeleton is taken into account: M-selectivity implies that one has no interest in mixing cycles
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of the game arenathat are read as cycles on the same memory state in the skeleton M.
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Let us give a quick breakdown of our paper. Due to space constraints, we only provide
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an intuitive overview of our results and technical approach in this conference version: formal
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details and proofs, along with additional results, can be found in the full article [6].
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In Section2, we introduce some basic notions, including the memory skeletons, and we
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establish several technical results. We also discussoptimal strategies andNash equilibria.
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In Section3, we introduceM-monotony and M-selectivity, cornerstones of our work. We
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also present two essential tools: prefix-coversandcyclic-coversof arenas. Section4states
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formally our characterization (Theorem 9), as well as the corresponding lifting corollary
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(Corollary10), from one-player to two-player games. We show an example of application
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in Section5. Finally, we give an overview of the technical highlights of our approach in
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Section6— its details are broken down in several intermediate results in our full paper [6].
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In a nutshell, the proof of the characterization (Theorem 9) is split in two. We first
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establish that (the sufficiency of) finite memory based on M implies M-monotony and
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M-selectivity of the preference relation. The crux is to build game arenas based on automata
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recognizing the languages involved in the two concepts, and to use the existence of finite-
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memory optimal strategies in these arenas to prove thatM-monotony andM-selectivity hold.
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To prove the converse implication, we proceed in two steps, first establishing the existence of
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memorylessoptimal strategies in “covered” arenas, and then building on it to obtain the
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existence offinite-memoryoptimal strategies in general arenas. The main technical tools we
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use are Nash equilibria and the aforementioned notions of prefix-covers and cyclic-covers.
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Alongside the technical details, we analyze our characterization in Appendix A: we
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highlight some limitations and interesting features, compare our techniques with Gimbert and
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Zielonka’s, discuss our place in the research landscape, and sketch directions for future work.
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Let us just stress already that our result — relating a memory skeletonMand preference
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relations for which this skeleton suffices — cannot be obtained by simply considering product
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arenas and invoking Gimbert and Zielonka’s result on memoryless determinacy [27]. While,
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of course, memoryless strategies on product arenas correspond to memoryfull strategies on
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original arenas (as we will formally establish in Lemma1), invoking [27] requires to be able
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to quantify onall arenas, not onlyproduct arenas. Filling this gap is exactly the goal of this
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paper, and it is made possible through the new concepts we sketched above.
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2 Preliminaries
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We only give here the notions and results necessary to understand this overview. Necessary
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notions and results — some of them interesting in their own right — to understand the
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technical details of the approach are found in the full paper [6].
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Automata and languages of colors. LetCbe an arbitrary set ofcolors. We assume knowl-
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edge ofnon-deterministic finite-state automata (NFA), which recognizeregular languages.
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For any finite subsetB⊆C, we denote byReg(B) the set of all regular languages overB.
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LetR(C) =S
B⊆C,|B|<∞Reg(B),that is, all the regular languages built overC.
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Let K⊆C∗ be a language offinitewords. We writePrefs(K) for the set of allprefixes of
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words inK. We define the set ofinfinite words [K] ={w=c1. . . ∈Cω| ∀n≥1, c1. . . cn∈
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Prefs(K)},which contains all infinite words for which every finite prefix is a prefix of a word
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inK. Intuitively, ifK is regular, [K] is the language of infinite words that correspond to
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infinite paths that can always branch and reach a final state, on an automaton forK. Given
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a finite wordw∈C∗ and a languageK⊆C∗, we writewK for their concatenation, i.e., the
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languagewK={w0=ww00|w00∈K} ⊆C∗.
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Arenas. We consider two players: player 1 (P1) and player 2 (P2). An arena is a tuple
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A= (S1, S2, E) such thatS =S1]S2(disjoint union) is a finite set ofstatespartitioned into
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states ofP1 (S1) andP2 (S2), andE⊆S×C×S is a finite set ofedges. Letcol:E→Cbe
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the projection of edges tocolorsandcolc its natural extension tosequences of edges. For an
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edgee∈E, we usein(e) andout(e) to denote its starting state and arrival state respectively,
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i.e.,e= (in(e),col(e),out(e)). We assume all arenas to be non-blocking, i.e., for all s∈S,
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there existse∈Esuch thatin(e) =s. Fori∈ {1,2}, we call an arenaA= (S1, S2, E) aPi’s
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one-player arena if for alls∈S3−i,|{e∈E|in(e) =s}|= 1 — that is,P3−i has no choice.
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Let Hists(A, s) denote the histories in A from s ∈ S, i.e., finite sequences of edges
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ρ= e1. . . en ∈E+ such that in(e1) = s and for alli, 1 ≤i < n, out(ei) = in(ei+1). Let
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Plays(A, s) denote theplays inAfroms∈S, i.e., infinite sequences π=e1e2. . . ∈Eω such
286
thatin(e1) =sand for alli≥1,out(ei) =in(ei+1). We write Hists(A, S0) andPlays(A, S0)
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for unions overS0 ⊆S, and writeHists(A) andPlays(A) for the unions over all states ofA.
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Let ρ=e1. . . en ∈Hists(A) (resp.π=e1e2. . .∈Plays(A)): we extend the operatorin
289
to histories (resp. plays) by identifyingin(ρ) (resp.in(π)) to in(e1). We proceed similarly
290
foroutand histories: out(ρ) =out(en). For the sake of convenience, we consider that any
291
set Hists(A, s) contains the empty history λs such that in(λs) = out(λs) = s. We write
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Histsi(A, s) andHistsi(A) for the subsets of historiesρsuch thatout(ρ)∈Si, i∈ {1,2}, i.e.,
293
histories whose last state belongs toPi. For any setH ⊆Hists(A), we writecol(H) for itsc
294
projection to colors, i.e.,col(Hc ) ={ccol(ρ)|ρ∈H}. We do the same for sets of plays.
295
Memory skeletons. Amemory skeleton is a tupleM= (M, minit, αupd) whereM is afinite
296
set ofstates,minit∈M is a fixedinitial state andαupd:M×C→M is anupdate function.
297
We writeαdupdfor the natural extension ofαupdto sequences inC∗. Memory skeletons are
298
deterministic and might have aninfinitenumber of transitions, in contrast to NFA. We define
299
thetrivial memory skeleton asMtriv= (M ={minit}, minit, αupd:{minit} ×C→ {minit}): it
300
permits to formalize memoryless strategies [27] in our framework.
301
Let M= (M, minit, αupd) be a skeleton. Form, m0∈M, we define the languageLm,m0 =
302
{w∈C∗|αdupd(m, w) =m0}that contains all words that can be read frommto m0 inM.
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LetM1= (M1, m1init, α1upd) andM2= (M2, m2init, α2upd). Theirproduct M1⊗ M2is the
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memory skeletonM= (M, minit, αupd) whereM =M1×M2, minit= (m1init, m2init), and, for
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allm1 ∈M1,m2∈M2,c∈C, αupd((m1, m2), c) = (α1upd(m1, c), α2upd(m2, c)). That is, the
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memories are updated in parallel when a color is read.
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Product arenas. LetA= (S1, S2, E) be an arena andM= (M, minit, αupd) be a skeleton.
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Their product AnM is the arena (S10, S20, E0) where S10 = S1×M, S20 = S2×M, and
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E0 ⊆S0×C×S0, withS0 =S10 ]S20, is such that ((s1, m1), c,(s2, m2))∈E0 if and only if
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(s1, c, s2)∈Eandαupd(m1, c) =m2. That is, the memory is updated according to the colors
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of the edges inE. Though Mmight contain an infinite number of transitions, AnM is
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always finite, asE is finite. Since we assumeAis non-blocking, it is also the case ofAnM.
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Strategies. A strategy σi for Pi, i ∈ {1,2}, on arena A = (S1, S2, E), is a function
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σi:Histsi(A)→E such that for allρ∈Histsi(A),in(σi(ρ)) =out(ρ). Let Σi(A) be the set
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of all strategies ofPion A. Afinite-memory strategy σi can be encoded as aMealy machine,
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i.e., a skeletonM= (M, minit, αupd) with transitions over afinite subset of colorsB ⊆C,
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enriched with anext-action function αnxt:M ×Si →E such that for all m∈M, s∈ Si,
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in(αnxt(m, s)) =s. Given a Mealy machine Γσi = (M, αnxt), strategyσi is defined as follows:
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∀s∈Si, σi(λs) =αnxt(minit, s),
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∀ρ·e∈Histsi(A),e∈E, σi(ρ·e) =αnxt
αdupd
minit,colc(ρ·e)
,out(e) .
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Let ΣFMi (A) be the finite-memory strategies ofPionA. We say that a strategyσi∈ΣFMi (A)
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isbased on memory skeleton Mif it can be encoded as a Mealy machine Γσi= (M, αnxt), as
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above. We implicitly assume that strategies of ΣFMi (A) are built by restricting the transitions
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of their skeletonMto the actual subset of colors appearing inA. A strategyσiismemoryless
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if it is a functionσi: Si→E, or equivalently, if it is based onMtriv.
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We writePlays(A, s, σi) for the playsconsistentwith a strategyσiofPifrom a states, i.e.,
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playsπ=e1e2. . .∈Plays(A, s) such that for allρ=e1. . . en,out(ρ)∈Si =⇒ σi(ρ) =en+1.
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We writePlays(A, s, σ1, σ2) for the singleton set containing the unique play consistent with a
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couple of strategies for the two players. We use similar notations for histories.
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Preference relations. Let v be a total preorder on Cω, called preference relation. We
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considerantagonistic games, where the objective of P1 is to create the best possible play
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with regard tovwhereas the objective of P2 is the opposite. That is,P2 uses the inverse
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relationv−1. This corresponds tozero-sum games when using a quantitative framework.
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Givenw, w0 ∈Cω, we writew@w0 if we have¬(w0 vw) since the preorder is total. We
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extendvto subsets: forW, W0 ⊆Cω,W vW0 ⇐⇒ ∀w∈W,∃w0 ∈W0, wvw0.We also
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writeW @W0 ⇐⇒ ∃w0∈W0, ∀w∈W, w@w0.Note thatW @W0 ⇐⇒ ¬(W0vW).
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To compare a wordw∈Cω with a languageK⊆Cω, we simply identify it to{w}.
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Games. A (deterministic turn-based two-player)game is a tupleG = (A,v) whereA is
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an arena andvis a preference relation. All classical objectives from the literature (both
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qualitative and quantitative) can be expressed in the general framework of preference relations
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(see Example 3 in [6]). Fori∈ {1,2}, aPi’s one-player gameis a gameG= (A,v) such that
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Ais a Pi’s one-player arena.
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Optimal strategies. Let G = (A,v) be a game on arena A = (S1, S2, E). Given a Pi-
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strategyσi∈Σi(A) and a states∈S, we define
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UColv(A, s, σi) ={w∈Cω| ∃σ3−i∈Σ3−i(A),col(Plays(A, s, σc 1, σ2))vw},
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DColv(A, s, σi) ={w∈Cω| ∃σ3−i∈Σ3−i(A), wvcol(Plays(A, s, σc 1, σ2))}.
347348
Note thatDColv(A, s, σi) =UColv−1(A, s, σi). Intuitively,UColv andDColv represent the
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upwardanddownward closuresof sequences of colors (consistent with a strategy) with respect
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to the preference relation.
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Taking the standpoint of P1, we say thatσ1∈Σ1(A) isat least as good as σ01∈Σ1(A)
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froms∈S ifUColv(A, s, σ1)⊆UColv(A, s, σ01). Intuitively,σ1 is at least as good asσ10 if
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the “worst-case” plays consistent withσ1 are at least as good as the ones consistent withσ10.
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TheUColoperator is useful to define this notion properly even in the case where there is no
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“worst-case” play (i.e., if the infimum used in the classical quantitative setting is not reached).
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Similar notions have been used before, e.g., in [38]. Symmetrically, for P2, we say that
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σ2∈Σ2(A) is at least as good asσ20 ∈Σ2(A) froms∈SifDColv(A, s, σ2)⊆DColv(A, s, σ02).
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Now, we say thatσi∈Σi(A) ofPi isoptimal froms∈S, akas-optimal, if it is at least
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as good as every otherσ0i∈Σi(A) froms. We extend this notation to subsets of states in
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the natural way, and we say that a strategyσi isuniformly-optimal if it isS-optimal.
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Our goal is to characterize the preference relations that admit uniformly-optimal finite-
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memory (UFM) strategies based on a given skeletonMin all arenas. We also discuss the
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simpler case ofuniformly-optimal memoryless (UML) strategies, which corresponds to the
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subcase studied by Gimbert and Zielonka [27], using the trivial skeletonMtriv.
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In that respect, the following link is important to observe.
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ILemma 1. Let G= (A,v)be a game on arena A= (S1, S2, E). Let M= (M, minit, αupd)
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be a memory skeleton and letσi∈ΣFMi (A)be a finite-memory strategy encoded by the Mealy
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machineΓσi = (M, αnxt). Then,σi is a UFM strategy in G if and only ifαnxt corresponds to
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an(S× {minit})-optimal memoryless strategy inG0= (AnM,v).
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Nash equilibria. We use Nash equilibria [36] as tools to establish the existence of optimal
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strategies in some of our proofs. LetG= (A,v) be a game on arenaA= (S1, S2, E). Formally,
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aNash equilibrium(NE) from a states∈Sis a couple of strategies (σ1, σ2)∈Σ1(A)×Σ2(A)
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such that, for allσ10 ∈Σ1(A),σ20 ∈Σ2(A),
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col(Plays(A, s, σc 10, σ2))vcol(Plays(A, s, σc 1, σ2))vcol(Plays(A, s, σc 1, σ02)). (1)
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Similarly to optimal strategies, we call an NEuniformif it is an NE from all states s∈S.
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The connection between optimal strategies andNash equilibria in our specific context of
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antagonisticgames is interesting to discuss, especially with respect to Gimbert and Zielonka’s
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original work [27]. We defer this discussion to [6] due to space constraints, and only provide
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here a brief account of the results one has to know to understand this overview. First, NE
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are de facto pairs of optimal strategies. Second, it is possible to mix different NE.
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ILemma 2. LetG= (A,v)be a game on arenaA= (S1, S2, E), and lets∈S be a state.
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Let (σ1a, σ2a)and(σb1, σb2)∈Σ1(A)×Σ2(A)be two Nash equilibria froms. Then,(σa1, σb2) is
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also a Nash equilibrium froms.
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IRemark 3. Lemma2crucially relies on the assumption (transparent in our definition of
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Nash equilibrium) that we considerantagonisticgames, that is,P2uses the inverse preference
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relationv−1. C
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3 Concepts
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Generalizing monotony and selectivity. As seen in Section 1, Gimbert and Zielonka’s
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characterization [27] relies onmonotony andselectivity of the preference relation. The main
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difference between their technical approach and ours is the following. In the memoryless
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setting, all the reasoning can be abstracted awayfrom the underlying arena and done on
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sequences of colors. In the finite-memory one, however, one has to pay attention to how
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sequences of colors are composed and compared, to maintain consistency with regard to
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the memory and the game arena. This need to intertwine abstract reasoning on arbitrary
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sequences of colors with concrete tracking of memory updates is the key obstacle to overcome.
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Much of our effort was thus spent on trying to define concepts that would preserve the
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elegance of monotony and selectivity while allowing us to lift the theory to the finite-memory
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case. As often the case, the good concepts turned out to be the most natural ones, capturing
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the intuitive idea that one needs monotony and selectivitymodulo a memory skeleton.
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IDefinition 4(M-monotony). LetM= (M, minit, αupd)be a memory skeleton. A preference
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relationvisM-monotoneif for allm∈M, for allK1, K2∈ R(C),
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(∃w∈Lminit,m,[wK1]@[wK2]) =⇒ (∀w0∈Lminit,m,[w0K1]v[w0K2]). (2)
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Recall that a skeletonMhas a fixed initial stateminit. Intuitively,M-monotonyextends
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Gimbert and Zielonka’s monotony by comparing prefixesbelonging to the same language
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Lminit,m, that is, prefixes that are deemed equivalent by skeletonM. This property roughly
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captures thatvisstable with regard to prefix addition, for memory-equivalent prefixes.
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The original monotony notion is equivalent to ourM-monotony withMbeing the trivial
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skeletonMtriv: that is, the memoryless case is naturally a subcase of our framework.
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IDefinition 5(M-selectivity). LetM= (M, minit, αupd)be a memory skeleton. A preference
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relationvisM-selectiveif for allw∈C∗,m=αdupd(minit, w), for allK1, K2∈ R(C)such
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that K1, K2⊆Lm,m, for all K3∈ R(C),
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[w(K1∪K2)∗K3]v[wK1∗]∪[wK2∗]∪[wK3]. (3)
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Similarly, M-selectivity extends Gimbert and Zielonka’s selectivity by asking one to
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compare sequences of colorsbelonging to the same language Lm,m, that is, sequences read as
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cycles on the memory skeleton. Note also that the memory statem should be consistent
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with the prefixwread from the initial memory stateminit. This property roughly captures
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thatvisstable with regard to cycle mixing, for memory-equivalent cycles.
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Again, the original selectivity notion is exactly equivalent toMtriv-selectivity.
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In a nutshell,M-monotony deals with prefixes up to the first cycle (on memory) and
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M-selectivity deals with the cycles thereafter; we will see that memory skeletons can be built
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in a compositional way based on these two orthogonal yet complementary tasks.
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Our notions respect the natural intuition that access to additional memory should always
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be helpful: if a skeletonMis sufficient to classify sequences of colors in a way that guarantees
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M-monotony andM-selectivity, then it should also be the case for “more powerful” skeletons.
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ILemma 6. Let M and M0 be two memory skeletons. If v is M-monotone (resp. M-
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selective) then, it is also (M ⊗ M0)-monotone (resp.(M ⊗ M0)-selective).
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Prefix-covers and cyclic-covers. While the concepts of M-monotony and M-selectivity
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are the primordial ones for stating the characterization, we still need two additional notions
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to prove it. Let us sketch the issue. To prove that monotone and selective preference relations
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yield UML strategies, Gimbert and Zielonka deploy aninductive argument on the number
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of choices in an arena. Intuitively, we want to use a similar approach for UFM strategies,
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but because of the unavoidable coupling between the memory skeleton and the arena (e.g.,
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Lemma1), the induction argument breaks, as adding one choice in the arena results in adding
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many in theproduct arena (as many as there are memory states), where the reasoning needs
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to take place. New insight and techniques are thus needed to patch this scheme.
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To solve this issue, we decouple the two aspects. We first establish that, on arenas that
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inherently share the same good properties as product arenas (i.e., they already “classify”
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prefixes and cycles as the memory would), we can deploy the induction argument and obtain
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UML strategies. Then, we obtain UFM strategies on general arenas as a corollary. The crux
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is identifying such “good” arenas: this is done through the following notions.
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I Definition 7 (Prefix-covers and cyclic-covers). Let M = (M, minit, αupd) be a memory
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skeleton andA= (S1, S2, E)be an arena. LetScov⊆S.
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We say that Mis a prefix-cover ofScov in Aif for alls∈S, there existsms∈M such
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that, for all ρ∈Hists(A) such thatin(ρ)∈Scov,out(ρ) =s and such that for allρ0 proper
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prefix ofρ,out(ρ0)6=s, we have αdupd(minit,col(ρ)) =c ms.
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We say thatMis a cyclic-cover ofScovinAif for allρ∈Hists(A)such thatin(ρ)∈Scov,
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ifs=out(ρ)and m=αdupd(minit,col(ρ)), for allc ρ0∈Hists(A)such that in(ρ0) =out(ρ0) =s,
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αdupd(m,col(ρc 0)) =m.
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Intuitively,M is a prefix-cover for a set of statesScov if the histories starting inScov and
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visiting a given states∈S for the first time are read up to the same memory state in the
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memory skeleton. Similarly,Mis a cyclic-cover ofAif the cycles ofAare read as cycles in
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the memory skeleton, once the memory has been initialized properly.
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As hinted above, the canonical example of a prefix- and cyclic-covered arena is a product
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arena (but many more may be in this case; it is beneficial to be general with these concepts).
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I Lemma 8. Let M = (M, minit, αupd) be a memory skeleton and A = (S1, S2, E) be an
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arena. ThenM is a prefix- and cyclic-cover forScov=S× {minit} inAnM.
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4 Characterization
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Equivalence. We now have the necessary ingredients to state our equivalence result.
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ITheorem 9(Equivalence). Letvbe a preference relation and letMbe a memory skeleton.
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Then, both players have UFM strategies based on memory skeletonMin all gamesG= (A,v)
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if and only if vandv−1 areM-monotone andM-selective.
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We state this theorem broadly and with a focus on UFM strategies. The actual results
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we have for each direction of the equivalence — see [6, Section 4 and Section 5] — are a
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bit stronger, of wider applicability and/or more interesting, but this statement carries the
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take-home message of our work. It is also meant to mirror the seminal result of Gimbert
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and Zielonka [27, Theorem 2]: their result can be retrieved from Theorem9by taking the
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trivial memory skeletonMtriv. As such, our work brings astrict generalization of Gimbert
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and Zielonka’s results [27] to the finite-memory case.
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Lifting corollary. As discussed in Section 1, the work of Gimbert and Zielonka contains
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not one, buttwo great results. Alongside the aforementioned equivalence result, Gimbert
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and Zielonka provide a corollary of high practical interest [27, Corollary 7]: they essentially
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obtain as a by-product of their approach that if memoryless strategies suffice in all one-player
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games ofP1 and all one-player games ofP2, they also suffice in all two-player games.
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This provides an elegant way to prove that a preference relation (equivalently, an objective)
475
admits memoryless optimal strategieswithout proving monotony and selectivity at all: proving
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it in the two one-player subcases, which is generally much easier as it boils down to graph
477
reasoning, and then lifting the result to the general two-player case through the corollary.
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See examples of one-player vs. two-player complexity in [5,4,11].
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Again, we are able to lift this corollary to the arena-independent finite-memory case.
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