Planning with Expectation Models for Control
  In model-based reinforcement learning (MBRL), Wan et al. (2019) showed
conditions under which the environment model could produce the expectation of
the next feature vector rather than the full distribution, or a sample thereof,
with no loss in planning performance. Such expectation models are of interest
when the environment is stochastic and non-stationary, and the model is
approximate, such as when it is learned using function approximation. In these
cases a full distribution model may be impractical and a sample model may be
either more expensive computationally or of high variance. Wan et al.
considered only planning for prediction to evaluate a fixed policy. In this
paper, we treat the control case - planning to improve and find a good
approximate policy. We prove that planning with an expectation model must
update a state-value function, not an action-value function as previously
suggested (e.g., Sorg & Singh, 2010). This opens the question of how planning
influences action selections. We consider three strategies for this and present
general MBRL algorithms for each. We identify the strengths and weaknesses of
these algorithms in computational experiments. Our algorithms and experiments
are the first to treat MBRL with expectation models in a general setting.
