user: text: Write a concise one-sentence TLDR summary of the scientific paper content below.

Title: Actor-Attention-Critic for Multi-Agent Reinforcement Learning

Paper content:
Reinforcement learning in multi-agent scenarios is important for real-world applications but presents challenges beyond those seen in single-agent settings.
We present an actor-critic algorithm that trains decentralized policies in multi-agent settings, using centrally computed critics that share an attention mechanism which selects relevant information for each agent at every timestep.
This attention mechanism enables more effective and scalable learning in complex multi-agent environments, when compared to recent approaches.
Our approach is applicable not only to cooperative settings with shared rewards, but also individualized reward settings, including adversarial settings, and it makes no assumptions about the action spaces of the agents.
As such, it is flexible enough to be applied to most multi-agent learning problems
Reinforcement learning has recently made exciting progress in many domains, including Atari games , the ancient Chinese board game, Go , and complex continuous control tasks involving locomotion BID17 BID26 BID12 .
While most reinforcement learning paradigms focus on single agents acting in a static environment (or against themselves in the case of Go), real-world agents often compete or cooperate with other agents in a dynamically shifting environment.
In order to learn effectively in multi-agent environments, agents must not only learn the dynamics of their environment, but also those of the other learning agents present.To this end, several approaches for multi-agent reinforcement learning have been developed.
The simplest approach is to train each agent independently to maximize their individual reward, while treating other agents as part of the environment.
However, this approach violates the basic assumption underlying reinforcement learning, that the environment should be stationary and Markovian.
Any single agent's environment is dynamic and nonstationary due to other agents' changing policies.
As such, standard algorithms developed for stationary Markov decision processes fail.At the other end of the spectrum, all agents can be collectively modeled as a single-agent whose action space is the joint action space of all agents BID2 .
While allowing coordinated behaviors across agents, this approach is not scalable due to the action space size increasing exponentially with the number of agents.
It also demands a high degree of communication during execution, as the central policy must collect observations from and distribute actions to the individual agents.
In real-world settings, this demand can be problematic.Recent work BID20 attempts to combine the strengths of these two approaches.
In particular, a critic (or a number of critics) is centrally learned with information from all agents.
The actors, however, receive information only from their corresponding agents.
Thus, during testing, executing the policies does not require the knowledge of other agents' actions.
This paradigm circumvents the challenge of non-Markovian and non-stationary environments during learning.
Despite those progresses, however, algorithms for multi-agent reinforcement learning are still far from being scalable (to a larger number of agents) and being generically applicable to environments and tasks that are co-operative (sharing a global reward), competitive, or mixed.Our approach extends these prior works in several directions.
The main idea is to centrally learn a critic with an attention mechanism.
The intuition behind our idea is that in many real-world environ-ments, it is beneficial for agents to know what other agents it should pay attention to.
For example, a soccer defender needs to pay attention to attackers in their vicinity as well as the player with the ball, while she/he rarely needs to pay attention to the opposing team's goalie.
The specific attackers that the defender is paying attention to can change at different parts of the game, depending on the formation and strategy of the opponent.
A typical centralized approach to multi-agent reinforcement learning does not take these dynamics into account, instead simply considering all agents at all timepoints.
Our attention mechanism is able to dynamically select which agents to attend to at each time point, improving performance in multi-agent domains with complex interactions.The proposed approach has an input space linearly increasing with respect to the number of agents, as opposed to the quadratic increase in a previous approach BID20 .
It also works well in co-operative, competitive, and mixed environments, exceeding the capability of some prior work that focuses only on co-operative environments .We
have validated our approach on two simulated environments and tasks. We
plan to release the code for both the model and the environments after the reviewing period ends.The rest of the paper is organized as follows. In
section 2, we discuss related work, followed by a detailed description of our approach in section 3. We
report experimental studies in section 4 and conclude in section 5.
We propose an algorithm for training decentralized policies in multi-agent settings.
The key idea is to utilize attention in order to select relevant information for estimating critics.
We analyze the performance of the proposed approach with respect to the number of agents, different configurations of rewards, and the span of relevant observational information.
Empirical results are promising and we intend to extend to highly complicated and dynamic environments.
for j = 1 . . . num critic updates do Update target critic and policy parameters: DISPLAYFORM0 T update ← 0 Update critic: DISPLAYFORM1 2 , where Update policies: DISPLAYFORM2 DISPLAYFORM3 38: end function

assistant: text: We propose an approach to learn decentralized policies in multi-agent settings using attention-based critics and demonstrate promising results in environments with complex interactions.