Paper:
Continuous-time Value Function Approximation in Reproducing Kernel Hilbert Spaces
Motivated by the success of reinforcement learning (RL) for discrete-time tasks such as AlphaGo and Atari games, there has been a recent surge of interest in using RL for continuous-time control of physical systems (cf. many challenging tasks in OpenAI Gym and DeepMind Control Suite). Since discretization of time is susceptible to error, it is methodologically more desirable to handle the system dynamics directly in continuous time. However, very few techniques exist for continuous-time RL and they lack flexibility in value function approximation. In this paper, we propose a novel framework for model-based continuous-time value function approximation in reproducing kernel Hilbert spaces. The resulting framework is so flexible that it can accommodate any kind of kernel-based approach, such as Gaussian processes and kernel adaptive filters, and it allows us to handle uncertainties and nonstationarity without prior knowledge about the environment or what basis functions to employ. We demonstrate the validity of the presented framework through experiments.

Review:
Strengths 1. Considering dynamic programming problems in continuous time such that the methodologies and tools of dynamical systems and stochastic di_x000b_eren- tial equations is interesting, and the authors do a good job of motivating the generalities of the problem context.  2. The authors initially describe problems/methods in very general terms, which helps preserve understandability of the _x000c_first sections.  Weaknesses  1. The parameterizations considered of the value functions at the end of the day belong to discrete time, due to the need to discretize the SDEs and sample the state-action-reward triples. Given this discrete implementa- tion, and the fact that experimentally the authors run into the conven- tional di_x000e_culties of discrete time algorithms with continuous state-action function approximation, I am a little bewildered as to what the actual bene_x000c_t is of this problem formulation, especially since it requires a re- de_x000c_nition of the value function as one that is compatible with SDEs (eqn. (4) ). That is, the intrinsic theoretical bene_x000c_ts of this perspective are not clear, especially since the main theorem is expressed in terms of RKHS only.  2. In the experiments, the authors mention kernel adaptive _x000c_filters (aka kernel LMS) or Gaussian processes as potential avenues of pursuit for addressing the function estimation in continuous domains. However, these methods are fundamentally limited by their sample complexity bottleneck, i.e., the quadratic complexity in the sample size. There's some experimental ref- erence to forgetting factors, but this issue can be addressed in a rigorous manner that preserves convergence while breaking the bottleneck, see, e.g.,  A. Koppel, G. Warnell, E. Stump, and A. Ribeiro, ``Parsimonious on- line learning with kernels via sparse projections in function space," arXiv preprint arXiv:1612.04111, 2016.  Simply applying these methods without consideration for the fact that the sample size conceptually is approaching in_x000c_nity, makes an update of the form (16) inapplicable to RL in general. Evaluating the Bellman operator requires computing an expected value.  3. Moreover, the limited complexity of the numerical evaluation is reflective of this complexity bottleneck, in my opinion. There are far more effective RKHS value function estimation methods than GPTD in terms of value function estimation quality and memory efficiency:  A. Koppel, G. Warnell, E. Stump, P. Stone, and A. Ribeiro. ``Policy Evaluation in Continuous MDPs with E_x000e_cient Kernelized Gradient Tem- poral Di_x000b_fference," in IEEE Trans. Automatic Control (submitted), Dec. 2017."  It's strange that the authors only compare against a mediocre benchmark rather than the state of the art.  4. The discussion at the beginning of section 3 doesn't make sense or is written in a somewhat self-contradictory manner. The authors should take greater care to explain the di_x000b_erence between value function estimation challenges due to unobservability, and value function estimation problems that come up directly from trying to solve Bellman's evaluation equation. I'm not sure what is meant in this discussion.  5. Also, regarding L87-88: value function estimation is NOT akin supervised learning unless one does Monte Carlo rollouts to do empirical approxima- tions of one of the expectations, due to the double sampling problem, as discussed in  R. S. Sutton, H. R. Maei, and C. Szepesvari, \A convergent o(n) temporal- di_x000b_erence algorithm for o_x000b_-policy learning with linear function approxi- mation," in Advances in neural information processing systems, 2009, pp. 1609?1616.   and analyzed in great detail in :  V. R. Konda and J. N. Tsitsiklis, ``Convergence rate of linear two-timescale stochastic approximation," Annals of applied probability, pp. 796- 819, 2004.  6. The Algorithm 1 pseudo-code is strangely broad so as to be hand-waving. There's no speci_x000c_cs of a method that could actually be implemented, or even computed in the abstract. Algorithm 1 could just as well say "train a deep network" in the inner loop of an algorithm, which is unacceptable, and not how pseudo-code works. Specifically, one can't simply "choose" at random" an RKHS function estimation algorithm and plug it in and assume it works, since the lion-share of methods for doing so either re- quire in_x000c_nite memory in the limit or employ memory-reduction that cause divergence.  7. L107-114 seems speculative or overly opinionated. This should be stated as a remark, or an aside in a Discussion section, or removed.  8. A general comment: there are no transitions between sections, which is not good for readability.  9. Again, the experiments are overly limited so as to not be convincing. GPTD is a very simplistic algorithm which is not even guaranteed to pre- serve posterior consistency, aka it is a divergent Bayesian method. There- fore, it seems like a straw man comparison. And this comparison is con- ducted on a synthetic example, whereas most RL works at least consider a rudimentary OpenAI problem such as Mountain car, if not a real robotics, power systems, or _x000c_financial application.