Time Invariance

A system is called time invariance if and only if:

$x(t) --> [system] --> [time delay] --> y(t)\,$

yields the same result as

$x(t) --> [time delay] --> [system] --> y(t) \,$

Example of a time invariance system

System is: $f(x) = 23x \,$

$X_1(t) = t^2 \,$

$X_2(t) = 2t^2 \,$

$f(aX_1 + bX_2) = af(X_1) + bf(X_2) \,$

$f(at^2 + 2bt^2) = af(t^2) + bf(2t^2) \,$

$f(at^2 + 2bt^2) = a*23t^2 + b*46t^2 \,$

$f(at^2 + 2bt^2) = 23(at^2 + 2bt^2) \,$

$f(x) = 23x \,$

System is: $f(x) = 23x + 1\,$

$X_1(t) = t^2 \,$

$X_2(t) = 2t^2 \,$

$f(aX_1 + bX_2) \neq af(X_1) + bf(X_2) \,$

$f(at^2 + 2bt^2) \neq af(t^2) + bf(2t^2) \,$

$f(at^2 + 2bt^2) \neq a(23t^2+1) + b(23*(2t^2)+1) \,$

$f(at^2 + 2bt^2) \neq 23 at^2 + 1 + 46 bt^2 + b \,$

$f(at^2 + 2bt^2) \neq 23 (at^2 + 2bt^2) + a + b \,$

$f(x) \neq 23x + 1 \,$