Linearity
So a system is linear if its inputs x1(t), x2(t) or (x1[n], x2[n] for Discrete Time signals) yield outputs y1(t), y2(t) such as the response: a*x1(t)+b*x2(t) => a*y1(t)+b*y2(t).
Example: Linear
Example: Non-Linear
y[n] = 2*x[n]^3
x1[n] -> [sys] -> y1[n]=2*x1[n]^3 -> (X)*a +++
= a*2*x1[n]^3+2*b*x2[n]^3
x2[n] -> [sys] -> y2[n]=2*x2[n]^3 -> (X)*b +++
