A Causal Discrete time LSI system has an impulse response h[n], this is zero for n<0 and thus it is right sided.h[n]=0 for all

n< 0 for causality system function H(z).

Assuming H(z) has a non null ROC ,we require,

for n > 0,

thus must be included in the ROC.

Example:

Let for is causal,

but consider for causal.