# Time Invariance of a basic system?

1. Feb 5, 2013

### ElijahRockers

1. The problem statement, all variables and given/known data

I am supposed to determine wether or not the discrete time system

$x[n] \rightarrow y[n] = x[-n]$

is time invariant or not.

3. The attempt at a solution

Let $x_d[n] = x[n-n_0]$

$y_d[n] = x_d[-n] = x[-(n-n_0)] = x[-n+n_0]$

$y[n-n_0] = x[-(n-n_0)] = x[-n+n_0]$

Since $y_d[n] = y[n-n_0]$, shouldn't this prove time invariance?

The book says the answer is that it is not time invariant...

From the more qualitative definition, a time invariant system is one for which the behavior does not change depending on when it is evaluated...
Now, I see that for -ve values of n, the system looks ahead, and for +ve values of n the system looks behind. Would this be considered time variant because of this? If so, how do I go about showing that mathematically?

Last edited: Feb 5, 2013
2. Feb 5, 2013

### ElijahRockers

I have just run into a similar problem, where y[n] = Even{x[n-1]}.

When I try shifting the input, then shifting the output and comparing them, the expressions are equal, but the book is telling me the system is not time invariant.