Translations:Causality and stability of digital systems/17/en

Two cases are important. Case B1 is for $|a|<1$ (Figure 7a). In such a case, the region of convergence does not include the unit circle, and the anticausal signal $h_{k}$ is unstable. Case B2 is for $|a{\rm {|>}}1$ (Figure 7b). In that case, the region of convergence includes the unit circle, and the anticausal signal $h_{k}$ is stable.