The causality of a system was defined in section 2.2.4. From this definition, show that, for a linear time-invariant system, causality implies that the impulse response h[n] is zero for n < 0. One approach is to show that if h[n] is not zero n < 0, then the system cannot be causal. Show also that if the impulse response is zero for n < 0, then the system will necessarily be causal.