(d) explain why each signal above is either causal or not causal.

4. In each case I've described the input space X and system mapping S for some I/O system. All of the systems are linear, but none is time-invariant. For each system, find an input signal I EF* and a to ER for which S(Shiftt, (2)) + Shiftt, (S(z)). (a) X is the set of all decent signals and S(r) for every r e X has specification S6 0 if t 0 is given. 4. In each case I've described the input space X and system mapping S for some I/O system. All of the systems are linear, but none is time-invariant. For each system, find an input signal I EF* and a to ER for which S(Shiftt, (2)) + Shiftt, (S(z)). (a) X is the set of all decent signals and S(r) for every r e X has specification S6 0 if t 0 is given