= Oscillatory unit-root diagnosis from an undamped sample autocorrelation
{title2=$\Phi_{\rm osc}(B)=1-2\cos\omega\,B+B^2$}
A large, slowly damping oscillatory <sample autocorrelation function> suggests autoregressive zeros near a conjugate pair on the unit circle. For period $s$, the pair is near $e^{\pm2\pi i/s}$, with minimal real filter $1-2\cos(2\pi/s)B+B^2$. Such a plot is evidence, not proof: a finite sample cannot distinguish a near-unit stationary model or a random sinusoid solely from the shape.
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