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statistique, théorie et gestion de portefeuille - Docs at ISFA

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2.1. Bulles r<strong>at</strong>ionnelles multi-dimensionnelles <strong>et</strong> queues épaisses 41<br />

Q UANTITATIVE F INANCE Multi-dimensional r<strong>at</strong>ional bubbles<br />

f(k)<br />

f(1)<br />

0<br />

Figure A1. Schem<strong>at</strong>ic shape of the function f(κ)<strong>de</strong>fined in (A.23).<br />

where ρ(EPA [A]) is the spectral radius of A. By <strong>de</strong>finition,<br />

|| EPA [A] n <br />

|| | EPA [A] n<br />

xmax| =λ n max . (A.29)<br />

Then<br />

1<br />

lim<br />

n→∞ n ln EPA ||An<br />

1<br />

...A1|| lim ln ρ(EPA [A])n<br />

n→∞ n<br />

= ln ρ(EPA [A]) . (A.30)<br />

Now, suppose th<strong>at</strong> ρ(EPA [A]) 1. We obtain<br />

1<br />

f(1) = lim<br />

n→∞ n ln EPA ||An ...A1|| 0, (A.31)<br />

which is in contradiction with the necessary condition (A.24).<br />

Thus,<br />

References<br />

f(1)

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