A system is linear if for specific inputs $ x_1(t) $ and $ x_2(t) $ yielding the outputs $ y_1(t) $ and $ y_2(t) $, respectively, the combination $ ax_1(t) + bx_2(t) $, where $ a $ and $ b $ are any complex numbers, yields the output $ ay_1(t) + by_2(t) $.
