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statistique, théorie et gestion de portefeuille - Docs at ISFA

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276 9. Mesure <strong>de</strong> la dépendance extrême entre <strong>de</strong>ux actifs financiers<br />

E.3 Proof of lemma 2<br />

We want to prove th<strong>at</strong>, assuming ν > 0 and x0 > 1,<br />

The change of variable<br />

gives<br />

1<br />

ɛ<br />

∞<br />

1<br />

dx 1<br />

x ν<br />

1<br />

lim<br />

ɛ→0 ɛ<br />

<br />

1 + 1<br />

ν<br />

∞<br />

1<br />

Cν<br />

x−x0<br />

ɛ<br />

Consi<strong>de</strong>r the second integral. We have<br />

which allows us to write<br />

so th<strong>at</strong><br />

<br />

<br />

<br />

<br />

<br />

∞<br />

x 0<br />

ɛ<br />

du<br />

1<br />

dx 1<br />

x ν<br />

2 ν+1<br />

2<br />

1<br />

<br />

1 + 1<br />

ν<br />

u =<br />

=<br />

(1 + u2 ) ν+1<br />

2<br />

(1 + ɛu<br />

x0 )ν<br />

Cν<br />

(1 + u2 ) ν+1<br />

2<br />

The next step of the proof is to show th<strong>at</strong><br />

L<strong>et</strong> us calcul<strong>at</strong>e<br />

<br />

<br />

x0 ɛ<br />

du<br />

<br />

1−x 0<br />

ɛ<br />

1<br />

(1 + ɛu<br />

x0 )ν<br />

x 0<br />

ɛ<br />

1−x 0<br />

ɛ<br />

Cν<br />

du<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

1<br />

(1 + ɛu<br />

x0 )ν<br />

<br />

<br />

<br />

− 1<br />

=<br />

x − x0<br />

ɛ<br />

Cν<br />

x−x0<br />

ɛ<br />

∞<br />

1−x 0<br />

ɛ<br />

= 1<br />

xν ∞<br />

1−x0 0 ɛ<br />

= 1<br />

x ν 0<br />

2 ν+1<br />

2<br />

= 1<br />

xν . (E.117)<br />

0<br />

, (E.118)<br />

x 0<br />

ɛ<br />

1−x0 ɛ<br />

+ 1<br />

xν ∞<br />

x0 0 ɛ<br />

1<br />

du<br />

(ɛu + x0) ν<br />

du<br />

du<br />

du<br />

1<br />

(1 + ɛu<br />

x0 )ν<br />

1<br />

(1 + ɛu<br />

x0 )ν<br />

1<br />

(1 + ɛu<br />

x0 )ν<br />

Cν<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

Cν<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

Cν<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

Cν<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

+<br />

(E.119)<br />

(E.120)<br />

. (E.121)<br />

u ≥ x0<br />

, (E.122)<br />

ɛ<br />

<br />

<br />

<br />

<br />

<br />

≤<br />

Cν<br />

ν ν+1<br />

2 ɛ ν+1<br />

x ν+1<br />

0<br />

≤ ν ν+1<br />

2 ɛ ν+1<br />

x ν+1<br />

= ν ν+1<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

<br />

<br />

<br />

<br />

<br />

x0 ɛ<br />

0<br />

2 ɛ ν<br />

x ν 0<br />

, (E.123)<br />

∞<br />

1<br />

∞<br />

x 0<br />

ɛ<br />

du<br />

Cν<br />

(1 + ɛu<br />

x0 )ν<br />

dv Cν<br />

(1 + v) ν<br />

(E.124)<br />

(E.125)<br />

= O(ɛ ν ). (E.126)<br />

1−x 0<br />

ɛ<br />

38<br />

du<br />

−→ 1 as ɛ −→ 0. (E.127)<br />

1<br />

(1 + ɛu<br />

x0 )ν<br />

Cν<br />

(1 + u2<br />

ν+1<br />

ν ) 2<br />

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