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I was going through my signal and system notes.they say $y(t)=x(t/2)$ is a non causal system? As non causal system depend on future inputs. how $t=t/2$ is future value of time? i could not understand how? could somebody please explain it to me?

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Because for negative values of $t$ you have, for example, $y(-2) = x(-1)$ which depends on a future value of $x(t)$ at $t=-1$ for the current value of $y(t)$ at $t=-2$.

Note that $t=-1$ represents a future time for $t=-2$.

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  • $\begingroup$ You beat me to it ... $\endgroup$ – Matt L. Jan 17 '18 at 10:56
  • $\begingroup$ we never miss these questions :-) $\endgroup$ – Fat32 Jan 17 '18 at 10:57
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HINT:

Use a negative time $t$ to see that $y(t)$ depends on future input values.

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