Autocovariance Matrix for Stationary Process is Variance by Autocorrelation Matrix
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Theorem
Let $S$ be a strictly stationary stochastic process giving rise to a time series $T$.
Let $\sequence {s_n}$ be a sequence of $n$ successive values of $T$:
- $\sequence {s_n} = \tuple {z_1, z_2, \dotsb, z_n}$
Let $\boldsymbol \Gamma_n$ denote the autocovariance matrix associated with $S$ for $\sequence {s_n}$.
Let $\mathbf P_n$ denote the autocorrelation matrix associated with $S$ for $\sequence {s_n}$.
Then:
- $\boldsymbol \Gamma_n = \sigma_z^2 \mathbf P_n$
where $\sigma_z^2$ denotes the variance of $S$.
Proof
From Autocorrelation of Strictly Stationary Stochastic Process:
- $\rho_k = \dfrac {\gamma_k} {\gamma_0}$
Then from Autocovariance at Zero Lag for Strictly Stationary Stochastic Process is Variance:
- $\gamma_0 = \sigma_z^2$
Hence:
- $\gamma_k = \sigma_z^2 \rho_k$
and the result follows.
$\blacksquare$
Sources
- 1994: George E.P. Box, Gwilym M. Jenkins and Gregory C. Reinsel: Time Series Analysis: Forecasting and Control (3rd ed.) ... (previous) ... (next):
- Part $\text {I}$: Stochastic Models and their Forecasting:
- $2$: Autocorrelation Function and Spectrum of Stationary Processes:
- $2.1$ Autocorrelation Properties of Stationary Models:
- $2.1.3$ Positive Definiteness and the Autocovariance Matrix
- $2.1$ Autocorrelation Properties of Stationary Models:
- $2$: Autocorrelation Function and Spectrum of Stationary Processes:
- Part $\text {I}$: Stochastic Models and their Forecasting: