title: Hellinger Distance Constrained Regression
abstract: This paper introduces an off-policy reinforcement learning method that uses Hellinger distance between sampling policy (from what samples were collected) and current policy (policy being optimized) as a constraint. 
  Hellinger distance squared multiplied by two is greater than or equal to total variation distance squared and less than or equal to Kullback-Leibler divergence, therefore a lower bound for expected discounted return for the new policy is improved compared to the lower bound for training with KL. 
  Also, Hellinger distance is less than or equal to 1, so there is a policy-independent lower bound for expected discounted return. 
  HDCR is capable of training with Experience Replay, a common setting for distributed RL when collecting trajectories using different policies and learning from this data centralized. 
  HDCR shows results comparable to or better than Advantage-weighted Behavior Model and Advantage-Weighted Regression on MuJoCo tasks using tiny offline datasets collected by random agents. On bigger datasets (100k timesteps) obtained by pretrained behavioral policy, HDCR outperforms ABM and AWR methods on 3 out of 4 tasks. 
authors: Egor Rotinov
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