Off-policy Distributional Q(λ): Distributional RL without Importance Sampling

We introduce off-policy distributional Q(λ), a new addition to the family of off-policy distributional evaluation algorithms. Off-policy distributional Q(λ) does not apply importance sampling for off-policy learning, which introduces intriguing interactions with signed measures. Such unique properties distributional Q(λ) from other existing alternatives such as distributional Retrace. We characterize the algorithmic properties of distributional Q(λ) and validate theoretical insights with tabular experiments. We show how distributional Q(λ)-C51, a combination of Q(λ) with the C51 agent, exhibits promising results on deep RL benchmarks.