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Nonlinear Equations - UFRJ

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142 [CH. 10: HOMOTOPY<br />

P n+1<br />

x i<br />

[N(F t, X i)]<br />

x i+1<br />

z t<br />

R<br />

t i<br />

t i+1<br />

Figure 10.1: The homotopy step. This picture is in projective space.<br />

For the picture in linear space, the reader can imagine that he stands<br />

at the origin. The points X i+1 , N(F ti+1 , X i ) and the origin are in<br />

the same complex line.<br />

Lemma 10.8. Assume the conditions of Theorem 10.5. For short,<br />

write β = β(F ti , X i ) and µ = µ(F ti , X i ). If<br />

D 3/2<br />

2 βµ ≤ a 0, (10.5)<br />

‖F t − F s ‖ ≤ ɛ 1<br />

, and (10.6)<br />

µ<br />

2ɛ 2<br />

Φ t,s (X) ≤<br />

∀s ∈ [t<br />

D 3/2 i , t i+1 ], (10.7)<br />

µ<br />

then the following estimates hold for all s ∈ [t i , t i+1 ]:<br />

D 3/2<br />

µ(f s , x i )<br />

β(F s , X i )<br />

β(F s , X i )<br />

≤<br />

≤<br />

≥<br />

µ<br />

1 − ɛ 1<br />

(10.8)<br />

2 (1 − ɛ 1 )α<br />

D 3/2 µ<br />

(10.9)<br />

2 (ɛ 2 − a 0 ) √ 1 − ɛ 2 1<br />

D 3/2 (1 + ɛ 1 )µ<br />

(10.10)<br />

2 β(F s, X i )µ(f s , x i ) ≤ α (10.11)

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