Inverse Z-transform
$ x[n] = \oint_C {X(Z)}{Z ^ (n-1)} , dZ \ $
where C is a closed counterwise countour inside the ROC of the Z- transform and around the origin.
$ = \sum_{poles a_i} ( X(Z) Z ^ (n-1)) Residue ( X(Z) Z ^ (n-1)) \ $ $ = \sum_{poles a_i} $
