# Terminology

Reinforcement Learning is about learning a mapping from states to a probability distribution over actions. This is called the policy.

Policy = p(s,a) = probability of taking action a when in state s

S = set of all states (assume finite)

st = state at time t

A(st) = set of all possible actions given agent is in state st e S

at = action at time t

rt e R (reals) = reward at time t

At each timestep t=1,2,3,...

• the agent finds itself in a state st e S and
• on that basis chooses an action at e A(st).
• One timestep later, the agent receives a reward rt+1 and
• finds itself in a new state st+1.

The return, rett, is the total reward received starting at time t+1:

rett= rt+1 + rt+2 + rt+3 .... + rf

where rf is the reward at the final time step (can be infinite)

and the discounted return is

rett= rt+1 + g rt+2 + g2 rt+3 ....

where 0 <= g <= 1 is called the discount factor.

We assume that the number of states and actions is finite. We then define the state transition probabilities to be:

This is just the probability of transitioning from state s to state s' when action a has been taken.

Expected Rewards

The value function for policy p is

The action-value function for policy p is

Bellman's Equation for Vp(s) (Recursion on Vp(s)) is

### Bellman Optimality Equations

Goal: Find the policy that gives the greatest return over the long run. We say a policy p is better than or equal to policy p' if Vp(s) >= Vp'(s) for all s. There is always at least one such policy. Such a policy it is called an optimal policy and is denoted by p*. Its corresponding value function is called V*:

V*(s) = Vp*(s) = max_p Vp(s) , for all s

and the optimal action-value function

Q*(s,a) = Qp*(s,a) = max_p Qp(s,a) , for all s, a

The Bellman optimality equation is then

This equation has a unique solution. It is a system of equations with |S| equations and |S| unknowns. If P and R were known then, in principle, it can be solved using some method for solving systems of nonlinear equations. Once V* is known, the optimal policy is determined by always choosing the action that produces the largest V*.

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