The input $x(t)$ and the output $y(t)$ of a system are related as
\[
y(t)=e^{-t} \int_{-\infty}^{t} e^{\tau} x(\tau) d \tau, \quad-\infty < t < \infty. \]
The system is
- nonlinear.
- linear and time-invariant.
- linear but not time-invariant.
- noncausal.