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CHAPTER 4. THEORY 91<br />

Thisshowsthatifwechoosex and y closetogether,wehavethat<br />

Kmax<br />

|G2(u)(x, k) 2 − G2(u)(y, k) 2 |dk ≤ γ.<br />

−Kmax<br />

So Kmax<br />

−Kmax G2(u)(x, k) 2 is a continuous function. This means that G1(u),G2(u) ∈ X<br />

for u ∈ X, and therefore, Z maps X to X.<br />

Show Z is continuous<br />

Let {un} ⊂X with un → u ∈ X. To show that Z is continuous, we need to show<br />

Z(un) → Z(u), or G1(un) → G1(u) andG2(un) → G2(u). Now for the G1 term we<br />

have for k>0,<br />

|G1(u)(x, k) − G1(un)(x, k)| =<br />

<br />

<br />

C<br />

k<br />

x<br />

0<br />

dze C(z−x)<br />

k h(z,k) ¯<br />

Kmax<br />

−Kmax<br />

x<br />

≤ dz<br />

0<br />

C C(z−x)<br />

e k |<br />

k ¯ Kmax<br />

h(z,k)|<br />

−Kmax<br />

x<br />

≤ dz<br />

0<br />

C C(z−x)<br />

e k <br />

k ¯ <br />

h∞ 2KmaxuX<br />

= ¯ <br />

Cx<br />

−<br />

h∞ 2Kmaxu − unX 1 − e k<br />

≤ ¯ <br />

h∞ 2Kmaxu − unX,<br />

[u(z,k ′ ) − un(z,k ′ )]dk ′<br />

|u(z,k ′ ) − un(z,k ′ )|dk ′

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