In Assignment 2, you have shown that the autocorrelation function for the AR(2) process
x_t = (7/10)x_(t-1) - (1/10)x_(t-2) + Z_t, {Z_t}_(t in Z) ∼ WN(0, σ^2)
is given by
ρ(h) = (16/11)(1/2)^|h| - (5/11)(1/5)^|h|, h = 0, ±1, ±2, ...
(a) Derive the normalized spectral density function of {x_t}_(t in Z).
(b) Write down the power spectral density function of {x_t}_(t in Z).
(c) Plot the normalized spectral density and comment on its behavior.
Given the spectral density function
f(ω) = (1/π)(62 - 70cos(ω) + 12cos(2ω)), ω in (0, π)
compute the autocovariance function γ(k) and autocorrelation function ρ(k) of the underlying stochastic process, where k in Z.
Hint: Begin with computing the autocovariance function γ(k) at lag k = 0, then at k = 1,
k = 2 and finally k > 2.