Abstract

Distributional reinforcement learning (RL) has proven useful in multiple benchmarks as it enables approximating the full distribution of returns and extracts rich feedback from environment samples. The commonly used quantile regression approach to distributional RL -- based on asymmetric \(L_1\) losses -- provides a flexible and effective way of learning arbitrary return distributions. In practice, it is often improved by using a more efficient, asymmetric hybrid \(L_1\)-\(L_2\) Huber loss for quantile regression. However, by doing so, distributional estimation guarantees vanish, and we empirically observe that the estimated distribution rapidly collapses to its mean. Indeed, asymmetric \(L_2\) losses, corresponding to expectile regression, cannot be readily used for distributional temporal difference. Motivated by the efficiency of \(L_2\)-based learning, we propose to jointly learn expectiles and quantiles of the return distribution in a way that allows efficient learning while keepi

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