Complex z plane 00 X(z)=∑x[n]zn Region Of Convergence n=-o0 (ROC) 乎m Z-plane unit circle --Z< ejo -π≤0<π =1 1 e 1010 Complex z plane ( )    =− − =  n n X z x n z j unit circle Z = e −      Z =1 Region Of Convergence (ROC)