Markov Decision Process
A sequential decision problem for a fully observable, stochastic environment with a Markovian transition model and additive rewards is called a Markov decision process. It consists of a set of states, with initial state \(s_0\), a set \(ACTIONS(s)\) of actions in each state, a transition model \(P(s'|s, a)\), and a reward function \(R(s)\).
A policy, denoted \(\pi\), specifies what the agent should do in any state \(s\). This action is denoted by \(\pi(s)\). The optimal policy \(\pi^*\) yields the highest expected utility.
The careful balancing of risk and reward is a characteristic of MDPs that does not arise in deterministic search problems.
Optimality in Markov Decision Processes
Finite Horizon
\begin{equation} E\left( \sum_{t=0}^{h} r_t \right) \end{equation}
Infinite Horizon
\begin{equation} E\left( \sum_{t=0}^{\infty} \gamma^t r_t \right) \end{equation}
Average-reward
\begin{equation} \lim_{h \rightarrow \infty} E\left( \sum_{t=0}^{h} \frac{1}{h} r_t \right) \end{equation}
Learning Performance Kaelbling, Littman, and Moore, n.d.
- Asymptotic convergence:
\begin{equation} \pi_n \rightarrow \pi^* \text { as } n \rightarrow \infty \end{equation}
- PAC:
\begin{equation} P(N_{errors} > F(\cdot, \epsilon, \delta)) \le \delta \end{equation}
Does not give any guarantee about the policy while it is learning
- Regret (e.g. bound \(B\) on total regret):
\begin{equation} \mathrm{max} \sum_{t=0}^{T} r_{tj} - r_t < B \end{equation}
No notion of “many small mistakes”, or “few major mistakes”.
- Uniform-PAC
unifies notion of PAC and regret Dann, Lattimore, and Brunskill, n.d.