Causality for Systems with Rational System Functions H()=M2A +bM-1 M-1 +612+b0 N-1 N 1x+ 儿 No poles at o,ifM≤N A DT LTI System with rational system function H() is causal e(a) the roc is the exterior of a circle outside the outermost pole and (b)if we write H(E) as a ratio of polynomials en degree N(a< degree D(zCausality for Systems with Rational System Functions A DT LTI system with rational system function H(z) is causal ⇔ (a) the ROC is the exterior of a circle outside the outermost pole; and (b) if we write H(z) as a ratio of polynomials then