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Title: ASYNCHRONOUS MULTI-AGENT GENERATIVE ADVERSARIAL IMITATION LEARNING

Paper content:
Imitation learning aims to inversely learn a policy from expert demonstrations, which has been extensively studied in the literature for both single-agent setting with Markov decision process (MDP) model, and multi-agent setting with Markov game (MG) model.
However, existing approaches for general multi-agent Markov games are not applicable to multi-agent extensive Markov games, where agents make asynchronous decisions following a certain order, rather than simultaneous decisions.
We propose a novel framework for asynchronous multi-agent generative adversarial imitation learning (AMAGAIL) under general extensive Markov game settings, and the learned expert policies are proven to guarantee subgame perfect equilibrium (SPE), a more general and stronger equilibrium than Nash equilibrium (NE).
The experiment results demonstrate that compared to state-of-the-art baselines, our AMAGAIL model can better infer the policy of each expert agent using their demonstration data collected from asynchronous decision-making scenarios (i.e., extensive Markov games).
Imitation learning (IL) also known as learning from demonstrations allows agents to imitate expert demonstrations to make optimal decisions without direct interactions with the environment.
Especially, inverse reinforcement learning (IRL) (Ng et al. (2000) ) recovers a reward function of an expert from collected demonstrations, where it assumes that the demonstrator follows an (near-)optimal policy that maximizes the underlying reward.
However, IRL is an ill-posed problem, because a number of reward functions match the demonstrated data (Ziebart et al. (2008; ; Ho & Ermon (2016) ; Boularias et al. (2011) ), where various principles, including maximum entropy, maximum causal entropy, and relative entropy principles, are employed to solve this ambiguity (Ziebart et al. (2008; ; Boularias et al. (2011); Ho & Ermon (2016) ; Zhang et al. (2019) ).
Going beyond imitation learning with single agents discussed above, recent works including Song et al. (2018) , Yu et al. (2019) , have investigated a more general and challenging scenario with demonstration data from multiple interacting agents.
Such interactions are modeled by extending Markov decision processes on individual agents to multi-agent Markov games (MGs) (Littman & Szepesvári (1996) ).
However, these works only work for synchronous MGs, with all agents making simultaneous decisions in each turn, and do not work for general MGs, allowing agents to make asynchronous decisions in different turns, which is common in many real world scenarios.
For example, in multiplayer games (Knutsson et al. (2004) ), such as Go game, and many card games, players take turns to play, thus influence each other's decision.
The order in which agents make decisions has a significant impact on the game equilibrium.
In this paper, we propose a novel framework, asynchronous multi-agent generative adversarial imitation learning (AMAGAIL): A group of experts provide demonstration data when playing a Markov game (MG) with an asynchronous decision-making process, and AMAGAIL inversely learns each expert's decision-making policy.
We introduce a player function governed by the environment to capture the participation order and dependency of agents when making decisions.
The participation order could be deterministic (i.e., agents take turns to act) or stochastic (i.e., agents need to take actions by chance).
A player function of an agent is a probability function: given the perfectly known agent participation history, i.e., at each previous round in the history, we know which agent(s) participated, it provides the probability of the agent participating in the next round.
With the general MG model, our framework generalizes MAGAIL (Song et al. (2018) ) from the synchronous Markov games to (asynchronous) Markov games, and the learned expert policies are proven to guarantee subgame perfect equilibrium (SPE) (Fudenberg & Levine (1983) ), a stronger equilibrium than the Nash equilibrium (NE) (guaranteed in MAGAIL Song et al. (2018) ).
The experiment results demonstrate that compared to GAIL (Ho & Ermon (2016) ) and MAGAIL (Song et al. (2018) ), our AMAGAIL model can better infer the policy of each expert agent using their demonstration data collected from asynchronous decision-making scenarios.
Imitation learning (IL) aims to learn a policy from expert demonstrations, which has been extensively studied in the literature for single agent scenarios (Finn et al. (2016) ; Ho & Ermon (2016) ).
Behavioral cloning (BC) uses the observed demonstrations to directly learn a policy (Pomerleau (1991); Torabi et al. (2018) ).
Apprenticeship learning and inverse reinforcement learning (IRL) ((Ng et al. (2000) ; Syed & Schapire (2008); Ziebart et al. (2008; Boularias et al. (2011) )) seek for recovering the underlying reward based on expert trajectories in order to further learn a good policy via reinforcement learning.
The assumption is that expert trajectories generated by the optimal policy maximize the unknown reward.
Generative adversarial imitation learning (GAIL) and conditional GAIL (cGAIL) incorporate maximum casual entropy IRL (Ziebart et al. (2010) ) and the generative adversarial networks (Goodfellow et al. (2014) ) to simultaneously learn non-linear policy and reward functions (Ho & Ermon (2016); Zhang et al. (2019) ; Baram et al. (2017) ).
A few recent studies on multi-agent imitation learning, such as MAGAIL (Song et al. (2018) and MAAIRL (Yu et al. (2019) ), model the interactions among agents as synchronous Markov games, where all agents make simultaneous actions at each step t.
These works fail to characterize a more general and practical interaction scenario, i.e., Markov games including turn-based games (Chatterjee et al. (2004) ), where agents make asynchronous decisions over steps.
In this paper, we make the first attempt to propose an asynchronous multi-agent generative adversarial imitation learning (AMAGAIL) framework, which models the asynchronous decision-making process as a Markov game and develops a player function to capture the participation dynamics of agents.
Experimental results demonstrate that our proposed AMAGAIL can accurately learn the experts' policies from their asynchronous trajectory data, comparing to state-of-the-art baselines.
Beyond capturing the dynamics of participation vs no-participation (as only two participation choices), our proposed player function Y (and AMAGAIL framework) can also capture a more general case 5 , where Y determines how the agent participates in a particular round, i.e., which action set A

assistant: text: This paper extends the multi-agent generative adversarial imitation learning to extensive-form Markov games.