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Ad hoc teamwork problem describes situations where an agent has to cooperate with previously unseen agents to achieve a common goal. For an agent to be successful in these scenarios, it has to have cooperative skills. One could implement cooperative skills into an agent by using domain knowledge (e.g., goals, roles, and protocols) to design the agent&#39;s behaviours. However, in complex domains, domain knowledge might not be available. Therefore, it is interesting to explore how to directly learn cooperative skills from data. In this work, we apply meta-reinforcement learning (meta-RL) formulation in the context of ad hoc teamwork problem. Our experiments show that such a method could produce cooperative agents in two cooperative environments with different cooperative circumstances.

AAMAS 2021

Learning to Cooperate with Unseen Agents Through Meta-Reinforcement Learning

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Ad hoc teamwork problem describes situations where an agent has to cooperate with previously unseen agents to achieve a common goal. For an agent to be successful in these scenarios, it has to have cooperative skills. One could implement cooperative skills into an agent by using domain knowledge (e.g., goals, roles, and protocols) to design the agent's behaviours. However, in complex domains, domain knowledge might not be available. Therefore, it is interesting to explore how to directly learn cooperative skills from data. In this work, we apply meta-reinforcement learning (meta-RL) formulation in the context of ad hoc teamwork problem. Our experiments show that such a method could produce cooperative agents in two cooperative environments with different cooperative circumstances.

poster

For intelligent conversational agents to speak about albums of images with users as humans do, they must be able to make sense of images as humans do. Computer vision methods can report directly observable information, but human beings care about more than the directly observable; they value holistic narratives that include affective and motivational evaluations, casual connections, and other inferred relationships from external knowledge. Drawing from theories in cognitive sensemaking and narrative coherence, we propose an approach for image sequence understanding that strives to generate and evaluate hypotheses about the relationships between people, events, and objects in images using commonsense knowledge, which are formed into a consistent network of hypotheses and observed facts via multi-objective optimization. The result is an enriched knowledge representation in the form of a knowledge graph which may later be used by a conversational agent.

Image Sequence Understanding through Narrative Sensemaking

Thompson sampling is a well-known approach for balancing exploration and exploitation in reinforcement learning. It requires the posterior distribution of value-action functions to be maintained; this is generally intractable for tasks that have a high dimensional state-action space. We derive a variational Thompson sampling approximation for DQNs which uses a deep network whose parameters are perturbed by a learned variational noise distribution. We interpret the successful NoisyNets method \citefortunato2018noisy as an approximation to the variational Thompson sampling method that we derive. Further, we propose State Aware Noisy Exploration (SANE) which seeks to improve on NoisyNets by allowing a non-uniform perturbation, where the amount of parameter perturbation is conditioned on the state of the agent. This is done with the help of an auxiliary perturbation module, whose output is state dependent and is learnt end to end with gradient descent. We hypothesize that such state-aware noisy exploration is particularly useful in problems where exploration in certain high risk states may result in the agent failing badly. We demonstrate the effectiveness of the state-aware exploration method in the off-policy setting by augmenting DQNs with the auxiliary perturbation module.

State-Aware Variational Thompson Sampling for Deep Q-Networks

We study the problem of spending a budget that is common to n agents. Agents submit demands to a central planner who uses the budget to fund a subset of them. The utility of an agent is the part of the budget spent on her own accepted demands. In a fair solution, the successful demands of each agent would represent a 1/n fraction of the budget. However, this is rarely possible because every demand is indivisible, i.e. either accepted in its entirety or rejected. We are interested in worst-case bounds on the largest proportion of the budget that is dedicated to the least funded agent. Our approach is not to solve the corresponding max min problem for every instance, but to tackle the problem from a higher level. The size of the largest demand compared to the budget and the number of agents, are two parameters that significantly influence how much the worst-off agent gets. We propose worst-case bounds on the best utility of the least funded agent for the class of instances where the number of agents and the most expensive demand are fixed to given values. A characterization of this quantity is provided for 1 and 2 agents. For more than 2 agents, %our results we propose lower and upper bounds that constitute a 14/15-approximation of the optimal value. Every existence result is complemented with a polynomial algorithm that builds a feasible solution satisfying our bounds.

Worst-case Bounds for Spending a Common Budget

Recent advances in the blockchain research have been made in two important directions. One is refined resilience analysis utilizing game theory to study the consequences of selfish behavior of users (miners), and the other is the extension from a linear (chain) structure to a non-linear (graphical) structure for performance improvements, such as IOTA and Graphcoin. The first question that comes to mind is ? what improvements that a blockchain system would see by leveraging these new advances. In this paper, we consider three major properties for a blockchain system: ?-partial verification, scalability, and finality-duration. We establish a formal framework and prove that no blockchain system can achieve ?-partial verification for any fixed constant ?, high scalability, and low finality-duration simultaneously. We observe that classical blockchain systems like Bitcoin achieves full verification (?=1) and low finality-duration, Ethereum 2.0 Sharding achieves low finality-duration and high scalability. We are interested in whether it is possible to partially satisfy the three properties.

A Game Theoretical Analysis of Non-Linear Blockchain System

In stochastic games, individuals need to make decisions in multiple states and transitions between states influence the dynamics of strategies significantly. In this work, by describing the dynamic process in stochastic game as a Markov chain and utilizing the transition matrix, we introduce a new method, named state-transition replicator dynamics, to obtain the replicator dynamics of a stochastic game. Based on our proposed model, we can gain qualitative and detailed insights into the influence of transition probabilities on the dynamics of strategies. We illustrate that a set of unbalanced transition probabilities can help players to overcome the social dilemmas and lead to mutual cooperation in a cooperation back state, even if the stochastic game has the same social dilemmas in each state. Moreover, we also present that a set of specifically designed transition probabilities can fix the expected payoffs of one player and make him lose the motivation to update his strategies in the stochastic game.

Modeling Replicator Dynamics in Stochastic Games Using Markov Chain Method

Ranking the participants of a tournament has applications in voting, paired comparisons analysis, sports and other domains. In this paper we introduce bipartite tournaments, which model situations in which two different kinds of entity compete indirectly via matches against players of the opposite kind; examples include education (students/exam questions) and solo sports (golfers/courses). In particular, we look to find rankings via chain  graphs, which correspond to bipartite tournaments in which the sets of adversaries defeated by the players on one side are nested with respect to set inclusion. Tournaments of this form have a natural and appealing ranking associated with them. We apply chain editing -- finding the minimum number of edge changes required to form a chain graph -- as a new mechanism for  tournament ranking. The properties of these rankings are investigated in a probabilistic setting, where they arise as maximum likelihood estimators, and through the axiomatic method of social choice theory. Despite some nice  properties, two problems remain: an important anonymity axiom is violated, and chain editing is NP-hard. We address both issues by relaxing the minimisation constraint in chain editing, and characterise the resulting ranking methods via  a greedy approximation algorithm.

Rankings for Bipartite Tournaments via Chain Editing

In recent years, machine learning (ML) models have been successfully applied in a variety of real-world applications. However, they are often complex and incomprehensible to human users. This can decrease trust in their outputs and render their usage in critical settings ethically problematic. As a result, several methods for explaining such ML models have been proposed recently, in particular for black-box models such as deep neural networks (NNs). Nevertheless, these methods predominantly explain model outputs in terms of inputs, disregarding the inner workings of the ML model computing those outputs. We present Argflow, a toolkit enabling the generation of a variety of ‘deep’ argumentative explanations (DAXs) for outputs of NNs on classification tasks.

Argflow: A Toolkit for Deep Argumentative Explanations for Neural Networks

In many risk-aware and multi-objective reinforcement learning settings, the utility of the user is derived from the single execution of a policy. In these settings, making decisions based on the average future returns is not suitable. For example, in a medical setting a patient may only have one opportunity to treat their illness. When making a decision, just the expected return -- known in reinforcement learning as the value -- cannot account for the potential range of adverse or positive outcomes a decision may have. Our key insight is that we should use the distribution over expected future returns differently to represent the critical information that the agent requires at decision time.

Distributional Monte Carlo Tree Search for Risk-Aware and Multi-Objective Reinforcement Learning

In this study, we leverage multi-objective reinforcement learning to create tunable agents, i.e., agents that can adopt a range of different behaviours according to the designer's preferences, without the need for retraining. We apply this technique to sequential social dilemmas, settings where there is inherent tension between individual and collective rationality. Learning a single fixed policy in such settings leaves one at a significant disadvantage if the opponents' strategies change after learning is complete. In our work, we demonstrate empirically that the tunable agents framework allows easy adaption between cooperative and competitive behaviours in sequential social dilemmas without the need for retraining, allowing a single trained agent model to be adjusted to cater for a wide range of behaviours and opponent strategies.

Tunable Behaviours in Sequential Social Dilemmas using Multi-Objective Reinforcement Learning

Continual learning aims to provide intelligent agents capable of learning multiple tasks sequentially with neural networks. In most settings of the current approaches, the agent starts from randomly initialized parameters and is optimized to master the current task regardless of the usefulness of the learned representation for future tasks. Moreover, each of the future tasks uses all the previously learned knowledge although parts of this knowledge might not be helpful for its learning. These cause interference among tasks, especially when the data of previous tasks is not accessible. In this paper, we propose a new method, named Self-Attention Meta-Learner (SAM), which learns a prior knowledge for continual learning that permits learning a sequence of tasks, while avoiding catastrophic forgetting. SAM incorporates an attention mechanism that learns to select the particular relevant representation for each future task. We empirically show that we can achieve a better performance than several state-of-the-art methods for continual learning by building on top of the selected representation learned by SAM. We also show the role of the meta-attention mechanism in boosting informative features corresponding to the input task and identifying the correct target in the task agnostic inference. Finally, we demonstrate that popular existing continual learning methods gain a performance boost when they adopt SAM as a starting point.

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Image Sequence Understanding through Narrative Sensemaking

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