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Title: Learning Gaussian Policies from Smoothed Action Value Functions

Paper content:
State-action value functions (i.e., Q-values) are ubiquitous in reinforcement learning (RL), giving rise to popular algorithms such as SARSA and Q-learning.
We propose a new notion of action value defined by a Gaussian smoothed version of the expected Q-value used in SARSA.
We show that such smoothed Q-values still satisfy a Bellman equation, making them naturally learnable from experience sampled from an environment.
Moreover, the gradients of expected reward with respect to the mean and covariance of a parameterized Gaussian policy can be recovered from the gradient and Hessian of the smoothed Q-value function.
Based on these relationships we develop new algorithms for training a Gaussian policy directly from a learned Q-value approximator.
The approach is also amenable to proximal optimization techniques by augmenting the objective with a penalty on KL-divergence from a previous policy.
We find that the ability to learn both a mean and covariance during training allows this approach to achieve strong results on standard continuous control benchmarks.
Model-free reinforcement learning algorithms often alternate between two concurrent but interacting processes: (1) policy evaluation, where an action value function (i.e., a Q-value) is updated to obtain a better estimate of the return associated with taking a specific action, and (2) policy improvement, where the policy is updated aiming to maximize the current value function.
In the past, different notions of Q-value have led to distinct but important families of RL methods.
For example, SARSA BID18 BID22 BID26 ) uses the expected Q-value, defined as the expected return of following the current policy.
Q-learning BID28 ) exploits a hard-max notion of Q-value, defined as the expected return of following an optimal policy.
Soft Q-learning BID7 and PCL BID14 both use a soft-max form of Q-value, defined as the future return of following an optimal entropy regularized policy.
Clearly, the choice of Q-value function has a considerable effect on the resulting algorithm; for example, restricting the types of policies that can be expressed, and determining the type of exploration that can be naturally applied.In this work we introduce a new notion of action value: the smoothed action value functionQ π .
Unlike previous notions, which associate a value with a specific action at each state, the smoothed Qvalue associates a value with a specific distribution over actions.
In particular, the smoothed Q-value of a state-action pair (s, a) is defined as the expected return of first taking an action sampled from a normal distribution N (a, Σ(s)), centered at a, then following actions sampled from the current policy thereafter.
In this way, the smoothed Q-value can also be interpreted as a Gaussian-smoothed or noisy version of the expected Q-value.We show that smoothed Q-values possess a number of interesting properties that make them attractive for use in RL algorithms.
For one, the smoothed Q-values satisfy a single-step Bellman consistency, which allows bootstrapping to be used to train a function approximator.
Secondly, for Gaussian policies, the standard optimization objective (expected return) can be expressed in terms of smoothed Q-values.
Moreover, the gradient of this objective with respect to the mean and covariance of the Gaussian policy is equivalent to the gradient and the Hessian of the smoothed Q-value function, which allows one to derive updates to the policy parameters by having access to the derivatives of a sufficiently accurate smoothed Q-value function.This observation leads us to propose an algorithm called Smoothie, which in the spirit of (Deep) Deterministic Policy Gradient (DDPG) BID21 BID11 , trains a policy using the derivatives of a trained (smoothed) Q-value function, thus avoiding the high-variance of stochastic updates used in standard policy gradient algorithms BID29 BID10 .
Unlike DDPG, which is well-known to have poor exploratory behavior BID7 , the approach we develop is able to utilize a non-deterministic Gaussian policy parameterized by both a mean and a covariance, thus allowing the policy to be exploratory by default and alleviating the need for excessive hyperparameter tuning.Furthermore, we show that Smoothie can be easily adapted to incorporate proximal policy optimization techniques by augmenting the objective with a penalty on KL-divergence from a previous version of the policy.
The inclusion of a KL-penalty is not feasible in the standard DDPG algorithm, but we show that it is possible with our formulation, and it significantly improves stability and overall performance.
On standard continuous control benchmarks, our results are competitive with or exceed state-of-the-art, especially for more difficult tasks in the low-data regime.
We have presented a new Q-value function,Q π , that is a Gaussian-smoothed version of the standard expected Q-value, Q π .
The advantage of usingQ π over Q π is that its gradient and Hessian possess an intimate relationship with the gradient of expected reward with respect to mean and covariance of a Gaussian policy.
The resulting algorithm, Smoothie, is able to successfully learn both mean and covariance during training, leading to performance that can match or surpass that of DDPG, especially when incorporating a penalty on divergence from a previous policy.The success ofQ π is encouraging.
Intuitively it may be argued that learningQ π is more sensible than learning Q π .
The smoothed Q-values by definition make the true reward surface smoother, thus possibly easier to learn; moreover the smoothed Q-values have a more direct relationship with the expected discounted return objective.
We encourage future work to further investigate these claims as well as techniques to apply the underlying motivations forQ π to other types of policies.A PROOF OF EQUATION FORMULA0 We note that similar identities for Gaussian integrals exist in the literature BID16 BID17 and point the reader to these works for further information.The specific identity we state may be derived using standard matrix calculus.
We make use of the fact that DISPLAYFORM0 and for symmetric A, ∂ ∂A ||v|| DISPLAYFORM1 We omit s from Σ(s) in the following equations for succinctness.
The LHS of FORMULA0 Meanwhile, towards tackling the RHS of FORMULA0 we note that DISPLAYFORM2 Thus we have DISPLAYFORM3

assistant: text: We propose a new Q-value function that enables better learning of Gaussian policies.