Translations:Causality and stability of digital systems/29/en

where the first 0 occurs at time index $k=0$. Because $|a{\rm {|>}}1$, this signal damps geometrically in the negative time direction. Thus we have no clear-cut choice, as we did when $|a|<1$. We cannot have both causality and stability; we must choose one or the other. The usual choice is equation 56, which gives us stability at the expense of causality.