Invertible
An LTI system is invertible when each distinct input to the system yields a distinct output. An invertible LTI system has an inverse system that is also LTI.
Examples
$ y(t) = (1/5)*x(t)^2 - 15 $ is not invertible because distinct inputs do not yield distinct outputs. For example, $ x(t)=1 $ and $ x(t)= -1 $ will yield the same values for y(t).
$ y(t) = (1/5)*x(t) - 15 $ is invertible because for each distinct value of x(t), there is a distinct value of y(t).
