(s ',t')
 (St- t)





 (S ,t 4 (O4ft4+++-- (S ,t)






 Figure 3-9 The grid axT




 Imf(Rk-1,j)I = lf(sk'.tj+ ) f(sktj) f(sk-1'tj+1) + f(sk-1,t )l

 = |f(o,tj+ ) f(O,tj) .

Putting everything together, we get
 n-1
 E |I (Ri,j) = r rf(o,t + ) f(0,t i) + Z IA (f)
 i,j j=1 i k i ,J
 j>O
 S Var[t,t']f(O,-) + Var[(o,t),(s 't )]

 = Mv (R),

since m l(R) = Ifl(s',t') If (s',t) Ifl(s,t') + If|(s,t)

- Ifl(s',t') Ifl(s',t) = [|lf(0,0) + Var[0,s']f( ,O) + Var[ot.]f(O,*)
+ Var[(oO)(s','()]) [If(0,o) + ar[ 'f(,O) + Var[o,t]f(O'.)
+ Var[(o,0),(s',t) f)] [Var[(o,o),(s't )]() Var[(o,),(s',t))