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statistique, théorie et gestion de portefeuille - Docs at ISFA

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C Composition of the mark<strong>et</strong> portfolio<br />

In this appendix, we <strong>de</strong>rive the rel<strong>at</strong>ionship b<strong>et</strong>ween the composition of the mark<strong>et</strong> portfolio and the composition<br />

of the optimal portfolio Π obtained by the minimiz<strong>at</strong>ion of the risks measured by ρα(n).<br />

C.1 Homogeneous case<br />

We first consi<strong>de</strong>r a homogeneous mark<strong>et</strong>, peopled with agents choosing their optimal portfolio with respect<br />

to the same risk measure ρα. A given agent p invests a fraction w0(p) of his wealth W (p) in the risk-free<br />

ass<strong>et</strong> and a fraction 1 − w0(p) in the optimal portfolio Π. Therefore, the total <strong>de</strong>mand Di of ass<strong>et</strong> i is the<br />

sum of the <strong>de</strong>mand Di(p) over all agents p in ass<strong>et</strong> i:<br />

Di = <br />

Di(p) , (114)<br />

p<br />

457<br />

= <br />

W (p) · (1 − w0(p)) · ˜wi , (115)<br />

p<br />

= ˜wi · <br />

W (p) · (1 − w0(p)) , (116)<br />

p<br />

where the ˜wi’s are given by (110). The aggreg<strong>at</strong>ed <strong>de</strong>mand D over all ass<strong>et</strong>s is<br />

D = <br />

Di, (117)<br />

i<br />

= <br />

˜wi · <br />

W (p) · (1 − w0(p)), (118)<br />

i<br />

p<br />

= <br />

W (p) · (1 − w0(p)). (119)<br />

p<br />

By <strong>de</strong>finition, the weight of ass<strong>et</strong> i, <strong>de</strong>noted by wm i , in the mark<strong>et</strong> portfolio equals the r<strong>at</strong>io of its capitaliz<strong>at</strong>ion<br />

(the supply Si of ass<strong>et</strong> i) over the total capitaliz<strong>at</strong>ion of the mark<strong>et</strong> S = Si. At the equilibrium,<br />

<strong>de</strong>mand equals supply, so th<strong>at</strong><br />

w m i = Si Di<br />

=<br />

S D = ˜wi. (121)<br />

Thus, <strong>at</strong> the equilibrium, the optimal portfolio Π is the mark<strong>et</strong> portfolio.<br />

C.2 H<strong>et</strong>erogeneous case<br />

(120)<br />

We now consi<strong>de</strong>r a h<strong>et</strong>erogenous mark<strong>et</strong>, <strong>de</strong>fined such th<strong>at</strong> the agents choose their optimal portfolio with<br />

respect to different risk measures. Some of them choose the usual mean-variance optimal portfolios, others<br />

prefer any mean-ρα efficient portfolio, and so on. L<strong>et</strong> us <strong>de</strong>note by Πn the mean-ρα(n) optimal portfolio<br />

ma<strong>de</strong> only of risky ass<strong>et</strong>s. L<strong>et</strong> φn be the fraction of agents who choose the mean-ρα(n) efficient portfolios.<br />

By normaliz<strong>at</strong>ion, <br />

n φn = 1. The <strong>de</strong>mand Di(n) of ass<strong>et</strong> i from the agents optimizing with respect to<br />

ρα(n) is<br />

Di(n) = <br />

W (p) · (1 − w0(p)) · ˜wi(n), (122)<br />

p∈Sn<br />

= ˜wi(n) <br />

W (p) · (1 − w0(p)), (123)<br />

p∈Sn<br />

33

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