# Lecture 03 (Jan 26, 2022)

### Autoregression Moldes (AR)

General linear time series is

$x_t = \mu + \sum_{i=0}^\infty \psi_i \epsilon_i$

for white noise $\epsilon$.

Assum $(x_t)$ is stationary. This means the variance $\text{Var}(x_t)<\infty$ . $\mathbb{E}x_t = \mu$. The variance is

$\gamma_0 = \text{Var}(x_t) = \sum_{i=0}^\infty \psi_i^2 \sigma^2$

In order for the series to converge by staitonary requirements, need $\psi_i$ to decay sufficiently fast.

\begin{aligned} \gamma_l &= \text{Cov}(x_t, x_{t-l})\\ &= \mathbb{E}[\sum_{i=0}^\infty \psi_i \epsilon_{t-1} \sum_{j=0}^\infty \psi_j (\epsilon_{t-l-j})] \\ &= \sum_{i=0}^\infty \sum_{j=0}^\infty \psi_i\psi_j \mathbb{E}(\epsilon_{t-i}\epsilon_{t-l-j}) \end{aligned}

$\mathbb{E}(\epsilon_{t-i}\epsilon_{t-l-j}) = \sigma^2 \delta(i,l+j)$. This means

$\gamma_l = \sum_{j=0}^\infty \psi_{l+j}\psi_j \sigma^2$

This means

$\rho_l = \frac{\gamma_l}{\gamma_0} = \frac{\sum_{j=0}^\infty \psi_j \psi_{j+l}}{\sum_{j=0}^\infty \psi_j^2}$

$AR(1)$ Model:

$x_t = \phi_0 + \phi_1x_{t-1} + \epsilon_t$

Assum $X_t = \phi_0 + \phi_1 x_{t-1} + \epsilon_t$ is stationary, then

$\mu = \mathbb{E}(x_t) = \phi_0 + \mathbb{E}(x_{t-1}) = \phi_0 + \phi_1 \mu \quad$

This means

$\mu = \frac{\phi_0}{1-\phi_1}$

Assum $\phi_1\neq 1$ since this case is just random walk. If we have non zero mean, we can rewrite the $AR(1)$ term as

\begin{aligned} &x_t = (1-\phi_1) \mu + \phi_1 x_{t-1} + \epsilon_t \\ &x_{t} - \mu = \phi_1(x_{t-1} - \mu) +\epsilon_t \end{aligned}

This is the debiasing procedure. Wite recursively:

\begin{aligned} x_t- \mu &= \phi_1(x_{t-1} - \mu) + \epsilon_t \\ &= \phi_1^2(x_{t-2} - \mu) + \phi_1\epsilon_{t-1} + \epsilon_t \\ &= \phi_1^3(x_{t-3}-\mu) + \phi_1^2\epsilon_{t-2} + \phi_1\epsilon_{t-1} + \epsilon_t \end{aligned}

This reduces to

$x_t = \sum_{i=0}^\infty \phi_1^i \epsilon_{t-i}$

AR(1) model is then a linear time series model.

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