1. Property of additivity. If an input Xl (t) produces an output yr (t), and an input X2(t) produces an output Y2(t), then an input M (t) + X2(t) produces an output Yl (t) + Y2(t) for all pairs of inputs Xl (t) and X2(t).
2. Property of homogeneity. If an input x(t) produces an output y(t), then an input a x(t) produces an output a y(t) for any input x(t) and any constant a.
The property of homogeneity does not imply the property of additivity and vice versa.
Linear systems can often be represented by linear differential equations or difference equations. Also, any continuous-time system is linear if its input—output relationship can be described by the ordinary linear differential equation