Quantitative analysis-Ch12-characterizing cycles

If we want to forecast a series, at a minimum we'd like its mean and its covariance structure(i.e., the covariances between
current and past values) to be stable over time, in which case we say that the series is covariance statinary.

quantifying stability of the covariance structure is a bit tricky, but tremendously important, and we do it using the
autocovariance function.
if the covariance structure is stable over time, as required by covariance stationarity, then the autocovariances depend
only on displacement, T, not on time t, and we write:
Y(t,T)=Y(T)