21. Consider the following four structures:

Let Fl9F2, andF3 be the corresponding component failure distributions;
each of which is assumed to be IFR (increasing failure rate). Let F be the system failure distribution. All components are independent.

(a) For which structures is F necessarily IFR if Fx = F2 = F3? Give reasons.

(b) For which structures is F necessarily IFR if F2 = F31 Give reasons.

(c) For which structures is F necessarily IFR if F1 ^ F2 ^ F3? Give reasons.