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Signals and Systems - Engineering

Q1:

Which one is a non-causal system?

A y(n) = x(2n)

B y(n) = x(n/2)

C y(n) = x(n/2) + x(n)

D all

ANS:A - y(n) = x(2n)

Because in (a) if you put n = 1 then y(n) = x(2) which shows output depend upon future value. While in (b), (c) input converted to Past or Present and past. Note that n cannot be in fraction so for x(n/2) you will put n = 2, 4, ... etc. not 1, 3, 5 .... etc.