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statistique, théorie et gestion de portefeuille - Docs at ISFA

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RESEARCH PAPER Q UANTITATIVE F INANCE V OLUME 2 (2002) 264–281<br />

quant.iop.org I NSTITUTE OF P HYSICS P UBLISHING<br />

Imit<strong>at</strong>ion and contrarian behaviour:<br />

hyperbolic bubbles, crashes and chaos<br />

A Corcos 1 , J-P Eckmann 2,3 , A Malaspinas 2 , Y Malevergne 4,5<br />

and D Sorn<strong>et</strong>te 4,6<br />

1 CRIISEA, Université <strong>de</strong> Picardie, BP 2716, 80027 Amiens, France<br />

2 Dépt. <strong>de</strong> Physique Théorique, Université <strong>de</strong>Genève, CH-1211 Genève 4,<br />

Switzerland<br />

3 Section <strong>de</strong> M<strong>at</strong>hém<strong>at</strong>iques, Université <strong>de</strong>Genève, CH-1211 Genève 4,<br />

Switzerland<br />

4 Labor<strong>at</strong>oire <strong>de</strong> Physique <strong>de</strong> la M<strong>at</strong>ière Con<strong>de</strong>nsée, CNRS UMR6622 and<br />

Université <strong>de</strong> Nice-Sophia Antipolis, BP 71, Parc Valrose,<br />

06108 Nice Ce<strong>de</strong>x 2, France<br />

5 Institut <strong>de</strong> Science Financière <strong>et</strong> d’Assurances—Université Lyon I, 43,<br />

Bd du 11 Novembre 1918, 69622 Villeurbanne Ce<strong>de</strong>x, France<br />

6 Institute of Geophysics and Plan<strong>et</strong>ary Physics and Department of Earth and<br />

Space Science, University of California, Los Angeles, CA 90095, USA<br />

Received 25 September 2001, in final form 26 November 2001<br />

Published 2 August 2002<br />

Online <strong>at</strong> stacks.iop.org/Quant/2/264<br />

Abstract<br />

Imit<strong>at</strong>ive and contrarian behaviours are the two typical opposite <strong>at</strong>titu<strong>de</strong>s of<br />

investors in stock mark<strong>et</strong>s. We introduce a simple mo<strong>de</strong>l to investig<strong>at</strong>e their<br />

interplay in a stock mark<strong>et</strong> where agents can take only two st<strong>at</strong>es, bullish or<br />

bearish. Each bullish (bearish) agent polls m ‘friends’ and changes her<br />

opinion to bearish (bullish) if (i) <strong>at</strong> least mρhb (mρbh) among the m agents<br />

inspected are bearish (bullish) or (ii) <strong>at</strong> least mρhh >mρhb (mρbb >mρbh)<br />

among the m agents inspected are bullish (bearish). The condition (i) ((ii))<br />

corresponds to imit<strong>at</strong>ive (antagonistic) behaviour. In the limit where the<br />

number N of agents is infinite, the dynamics of the fraction of bullish agents<br />

is d<strong>et</strong>erministic and exhibits chaotic behaviour in a significant domain of the<br />

param<strong>et</strong>er space {ρhb,ρbh,ρhh,ρbb,m}. A typical chaotic trajectory is<br />

characterized by intermittent phases of chaos, quasi-periodic behaviour and<br />

super-exponentially growing bubbles followed by crashes. A typical bubble<br />

starts initially by growing <strong>at</strong> an exponential r<strong>at</strong>e and then crosses over to a<br />

nonlinear power-law growth r<strong>at</strong>e leading to a finite-time singularity. The<br />

reinjection mechanism provi<strong>de</strong>d by the contrarian behaviour introduces a<br />

finite-size effect, rounding off these singularities and leads to chaos. We<br />

document the main stylized facts of this mo<strong>de</strong>l in the symm<strong>et</strong>ric and<br />

asymm<strong>et</strong>ric cases. This mo<strong>de</strong>l is one of the rare agent-based mo<strong>de</strong>ls th<strong>at</strong> give<br />

rise to interesting non-periodic complex dynamics in the ‘thermodynamic’<br />

limit (of an infinite number N of agents). We also discuss the case of a finite<br />

number of agents, which introduces an endogenous source of noise<br />

superimposed on the chaotic dynamics.<br />

264 1469-7688/02/040264+18$30.00 © 2002 IOP Publishing Ltd PII: S1469-7688(02)29456-7<br />

161

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