Editing Linear time-invariant system (section)

==== Causality ====
{{Main|Causal system}}
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A system is causal if the output depends only on present and past, but not future inputs.  A necessary and sufficient condition for causality is
<math display="block">h(t) = 0 \quad \forall t < 0,</math>

where <math>h(t)</math> is the impulse response.  It is not possible in general to determine causality from the [[two-sided Laplace transform]]. However, when working in the time domain, one normally uses the [[Laplace transform|one-sided Laplace transform]] which requires causality.