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```
%matplotlib inline
import matplotlib.pyplot as plt
import seaborn as sns
sns.set(color_codes=True)
from IPython.display import Image
```

approximate value function: parameterized function $\hat{v}(s, w) \approx v_\pi(s)$

- applicable to partially observable problems.

$s \to u$: $s$ is the state updated and $u$ is the update target that $s$'s estimated value is shifted toward.

We use machine learning methods and pass to them the $s \to g$ of each update as a training example. Then we interperet the approximate function they produce as an estimated value function.

not all function approximation methods are equally well suited for use in reinforcement learning:

- learn efficiently from incrementally acquired data: many traditional methods assume a static training set over which multiple passes are made.
- are able to handle nonstationary target functions.

which states we care most about: a state distribution $\mu(s) \geq 0$, $\sum_s \mu(s) = 1$.

- Often $\mu(s)$ is chosen to be the fraction of time spent in $s$.

objective function, the Mean Squared Value Error, denoted $\overline{VE}$:

\begin{equation} \overline{VE}(w) \doteq \sum_{s \in \delta} \mu(s) \left [ v_\pi (s) - \hat{v}(s, w) \right ]^2 \end{equation}where $v_\pi(s)$ is the true value and $\hat{v}(s, w)$ is the approximate value.

Note that best $\overline{VE}$ is no guarantee of our ultimate purpose: to find a better policy.

- global optimum.
- local optimum.
- don't convergence, or diverge.

SGD: well suited to online reinforcement learning.

\begin{align} w_{t+1} &\doteq w_t - \frac1{2} \alpha \nabla \left [ v_\pi(S_t) - \hat{v}(S_t, w_t) \right ]^2 \\ &= w_t + \alpha \left [ \color{blue}{v_\pi(S_t)} - \hat{v}(S_t, w_t) \right ] \nabla \hat{v}(S_t, w_t) \\ &\approx w_t + \alpha \left [ \color{blue}{U_t} - \hat{v}(S_t, w_t) \right ] \nabla \hat{v}(S_t, w_t) \\ \end{align}$S_t \to U_t$, is not the true value $v_\pi(S_t)$, but some, possibly random, approximation to it. (前面各种方法累计的value）:

- If $U_t$ is an unbiased estimate, $w_t$ is guaranteed to converge to a local optimum.
- Otherwise, like boostrappig target or DP target => semi-gradient methods. (might do not converge as robustly as gradient methods)
- significantly faster learning.
- enable learning to be continual and online.

state aggregation: states are grouped together, with one estimated value for each group.

For every state $s$, there is a real-valued feature vector $x(s) \doteq (x_1(s), x_2(s), \dots, x_d(s))^T$:

\begin{equation} \hat{v}(s, w) \doteq w^T x(s) \doteq \sum_{i=1}^d w_i x_i(s) \end{equation}Choosing features appropriate to the task is an important way of adding prior domain knowledge to reinforcement learing systems.

- Polynomials
- Fourier Basis: low dimension, easy to select, global properities
- Coarse Coding
- Tile Coding: convolution kernel?
- Radial Basis Functions

A good rule of thumb for setting the step-size parameter of linear SGD methods is then $\alpha \doteq (\gamma \mathbf{E}[x^T x])^{-1}$

+ANN, CNN

$w_{TD} = A^{-1} b$: data efficient, while expensive computation

nearest neighbor method

RBF function

more interested in some states than others:

- interest $I_t$: the degree to which we are interested in accurately valuing the state at time $t$.
- emphaisis $M_t$:

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