System Characterized By Linear Constant-Coefficient Differential Equations
$ \sum_{k=0}^{N}a_k\frac {d^ky(t)}{dt^k} = \sum_{k=0}^{M}b_k\frac {d^kx(t)}{dt^k} $
=$ Y(jw)=H(jw)X(jw), H(jw)=\frac{Y(jw)}{X(jw)} $
Example
$ \frac{d^2y(t)}{dt^2}+4\frac{dy(t)}{dt}+3y(t) = \frac{dx(t)}{dt}+2x(t) $
