Editing Linear time-invariant system (section)

==== Causality ====
{{Main|Causal system}}
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A discrete-time LTI system is causal if the current value of the output depends on only the current value and past values of the input.<ref>Phillips 2007, p. 508.</ref>   A necessary and sufficient condition for causality is
<math display="block">h[n] = 0 \ \forall n < 0,</math>
where <math>h[n]</math> is the impulse response.  It is not possible in general to determine causality from the Z transform, because the inverse transform is not unique{{dubious|date=September 2020}}.  When a [[region of convergence]] is specified, then causality can be determined.