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statistique, théorie et gestion de portefeuille - Docs at ISFA

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where α = (c ∗ ,d ∗ ). It can be consistently estim<strong>at</strong>ed by<br />

The coefficient K22 is given by<br />

ˆK11(1,1) = 1<br />

ĉ u u<br />

− ln2<br />

+<br />

ĉ2 dˆ<br />

dˆ<br />

1 T<br />

T ∑ ln<br />

i=1<br />

2<br />

ĉ xi xi<br />

, (97)<br />

dˆ<br />

dˆ<br />

ˆK11(1,2) = ˆK11(2,1) = c<br />

ĉ u u<br />

ln<br />

−<br />

d dˆ<br />

dˆ<br />

1 T <br />

ĉ<br />

xi xi<br />

T ∑ ln<br />

, (98)<br />

ˆ<br />

i=1 d dˆ<br />

2 ˆK11(2,2)<br />

ĉ<br />

= . (99)<br />

dˆ<br />

K22 = −E0<br />

which is consistently estim<strong>at</strong>ed by ˆK22 = ˆb −2 .<br />

∂ 2 ln f (x|b ∗ )<br />

∂b ∗2<br />

<br />

99<br />

= 1<br />

, (100)<br />

b∗2 Now, we have to calcul<strong>at</strong>e the two components of the vector ˜C12 = ˜C t 21 . Its first component is<br />

<br />

˜C12(1)<br />

∂ln f1(x|c<br />

= E1<br />

∗ ,d ∗ )<br />

∂c∗ · ∂ln f2(x|b∗ )<br />

∂b∗ <br />

, (101)<br />

<br />

1 u<br />

= E1 + ln<br />

c∗ d∗ <br />

u<br />

d∗ c∗ <br />

+ ln x x<br />

− ln<br />

d∗ d∗ <br />

x<br />

d∗ c∗ <br />

1 u<br />

· + ln<br />

b∗ d∗ <br />

− ln x<br />

d∗ <br />

,(102)<br />

<br />

1 u<br />

= + ln<br />

c∗ d∗ <br />

u<br />

d∗ c∗ <br />

1 u 1 u<br />

· + ln − + ln<br />

b∗ d c∗ d∗ <br />

u<br />

d∗ c∗ <br />

· E1 ln x<br />

d∗ <br />

<br />

1 u<br />

+ + ln<br />

b∗ d∗ <br />

· E1 ln x<br />

d∗ <br />

− E1 ln x<br />

d∗ <br />

x<br />

d∗ c∗ <br />

2 x<br />

− E1 ln<br />

d∗ <br />

2 x<br />

+ E1 ln<br />

d∗ <br />

x<br />

d∗ c∗ (103) .<br />

Some simple calcul<strong>at</strong>ions show th<strong>at</strong><br />

E1<br />

E1<br />

<br />

ln x<br />

d ·<br />

<br />

x<br />

c d<br />

<br />

2 x<br />

ln<br />

d ·<br />

<br />

x<br />

c d<br />

= 1 u<br />

+ ln<br />

c d ·<br />

<br />

u<br />

c <br />

+ E1 ln<br />

d<br />

x<br />

<br />

, (104)<br />

d<br />

2 u<br />

= ln<br />

d ·<br />

<br />

u<br />

c +<br />

d<br />

2<br />

c E1<br />

<br />

ln x<br />

<br />

2 x<br />

<br />

+ E1 ln , (105)<br />

d d<br />

which allows us to show th<strong>at</strong> the first and third terms cancel out, and it remains<br />

˜C12(1) = 1<br />

<br />

E1 ln<br />

c∗ x<br />

d∗ <br />

− ln u<br />

d∗ <br />

u<br />

d∗ c∗ <br />

E1 ln x<br />

d∗ <br />

− ln u<br />

d∗ <br />

. (106)<br />

The second component is<br />

˜C12(2) =<br />

<br />

∂ln f1(x|c<br />

E1<br />

∗ ,d ∗ )<br />

∂d∗ · ∂ln f2(x|b∗ )<br />

∂b∗ =<br />

<br />

,<br />

<br />

E1 −<br />

(107)<br />

c∗<br />

d∗ <br />

u<br />

1 +<br />

d∗ c∗ <br />

x<br />

−<br />

d∗ c∗ <br />

1 u<br />

· + ln<br />

b∗ d∗ <br />

− ln x<br />

d∗ =<br />

<br />

,<br />

−<br />

(108)<br />

c∗<br />

d∗ <br />

u<br />

1 +<br />

d∗ c∗ 1 u<br />

+ ln<br />

b∗ d∗ <br />

u<br />

− 1 +<br />

d∗ c∗ <br />

E1 ln x<br />

d∗ −<br />

<br />

<br />

1 u<br />

+ ln<br />

b∗ d∗ x<br />

E1<br />

d∗ c∗ <br />

+ E1 ln x<br />

d∗ <br />

x<br />

d∗ c∗ . (109)<br />

35

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