Abstract
In this paper, we study turn-based multiplayer quantitative non zero-sum games played on finite graphs with reachability objectives. In this framework each player aims at reaching his own goal as soon as possible. We focus on existence results for two solution concepts: Nash equilibrium and secure equilibrium. We prove the existence of finite-memory Nash (resp. secure) equilibria in n-player (resp. 2-player) games. For the case of Nash equilibria, we extend our result in two directions. First, we show that finite-memory Nash equilibria still exist when the model is enriched by allowing n-tuples of positive costs on edges (one cost by player). Secondly, we prove the existence of Nash equilibria in quantitative games with both reachability and safety objectives.
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Notes
The general case of reachability/safety objectives is handled in Sect. 3.3.1.
Note that the length is not defined as the number of vertices.
We will keep this convention through the article.
Note that player 1 has no choice in vertices C and D, that is, σ 1(hv) is necessarily equal to A for v∈{C,D}.
For qualitative games, we use the notion of payoff rather than the notion of cost since Win (resp. Lose) can be seen as a payoff of 1 (resp. 0) and the aim of the players is to maximize their payoffs.
Indeed when j>k, i.e. when player j has not reached his goal set, the coalition punishes him in the exact same way as Lemma 16 by preventing him from visiting his goal set.
Remark that ≾1 (or ≾2) is a kind of lexicographic order on \((\mathbb{N}\cup\{+\infty\}) \times (\mathbb{N} \cup\{+\infty\})\).
We are conscious that it is counterintuitive to use the particular value −1, but it is helpful in the proofs.
Notice that in the second case, when ρ does not visit \({\sf{Goal}}_{1}\) in \({\sf{Trunc}}_{d}(\mathcal{T})\), player 1 may reach his goal set in \(\mathcal{T}\) when deviating in this way, and this would be profitable for him in this game.
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Acknowledgements
This work has been partly supported by the ESF project GASICS and a grant from the National Bank of Belgium. The third author is supported by a grant from L’Oreal-UNESCO/F.R.S.-FNRS. The authors are grateful to Jean-François Raskin and Hugo Gimbert for useful discussions.
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Brihaye, T., Bruyère, V. & De Pril, J. On Equilibria in Quantitative Games with Reachability/Safety Objectives. Theory Comput Syst 54, 150–189 (2014). https://doi.org/10.1007/s00224-013-9495-7
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DOI: https://doi.org/10.1007/s00224-013-9495-7