Among the following, the differential equation(s) not representative of a linear dynamical system is/are ____.
(i) \[ \frac{d^2x}{dt^2} + 2t\frac{dx}{dt} + x(t) = u(t) \]
(ii) \[ \left(\frac{dx}{dt}\right)^2 + 2\frac{dx}{dt} + x(t) = u(t) \]
(iii) \[ \frac{d^2x}{dt^2} + 2\frac{dx}{dt} + x(t) = u(t) \]