9.20 This exercise is related to Example 9.7.

(i) It was shown that t , t ∈ Z2, is a TMD with respect to P(t) = P2(t), but not necessarily a TMD with respect to P(t) = P1(t). Is t , t ∈ Z2, a TMD with respect to P(t) = P3(t), or P(t) = P4(t)?

(ii) If we switch the roles of P1(t) and P2(t) [i.e., define, instead of (9.48),

t = Wt − E{Ws, s ∈ P1(t)}], is t , t ∈ Z2, a TMD with respect to P(t) = P2(t)?

(iii) Is there a general rule that you can draw from Example 9.7 and this exercise?