 Sample autocovariance function

For a known zero mean, ${\hat{\gamma }}_n(h)=n^{-1}{\sum }_{t=1}^{n-h}{Y}_t{Y}_{t+h}$ for $0\le h<n$. Weakly stationary process assumptions give expectation $(1-h/n)\gamma (h)$. Dividing by $n-h$ instead gives an unbiased estimate at each individual lag, although the resulting collection need not form a positive semidefinite covariance sequence.