17.4 Derive the autocorrelation function for the ARMA(2, 1) process
y_t = ϕ_1 y_{t-1} + ϕ_2 y_{t-2} + ε_t - θ_1 ε_{t-1}
that is, determine ρ_1, ρ_2, etc., in terms of ϕ_1, ϕ_2, and θ_1. Draw this autocorrelation function for ϕ_1 = .6, ϕ_2 = .3, and θ_1 = .9. Repeat for ϕ_1 = .6, ϕ_2 = .3, and θ_1 = -.9. Repeat for ϕ_1 = .6, ϕ_2 = -.3, and θ_1 = -.9.