Linear system
Linear system is a system that satisfies a principle of superpositon. For example, if sinusoid signal is input of a linear system, the frequency of output signal is not changed. Only amplitude and phase can be changed.
Example of Linear system
y1(t) = T{x1(t)}
y2(t) = T(x2(t))
W(t) a*T{x1(t)} + a*T{x(2)}
Y(t) = T{a*x1(t) + a*x2(t)}
If W(t) eaquals to Y(t), System T is linear system.
