A stationary process $X\left( t\right)$, $t\in \mathbb{Z},$ has spectral density function $$\begin{equation*} f\left( \omega \right) =\pi -\omega , \end{equation*}$$ for $\omega \in \left( 0,\pi \right)$. Provide answers to the following to two decimal places.

Part (a) Find the variance of $X(t)$ :


Part (b) Find the autocovariance function of $X(t)$ at lag :


Part (c) Find the autocovariance function of $X(t)$ at lag :