Journal of Financial Econometrics Advance Access published online on July 11, 2007
Journal of Financial Econometrics, doi:10.1093/jjfinec/nbm010
Copyright © The Author 2007. Published by Oxford University Press.
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Positivity Conditions for a Bivariate Autoregressive Volatility Specification
C. Gourieroux
CREST, and University of Toronto
Address correspondence to C. Gourieroux, CREST, and University of Toronto, Canada, UK, or e-mail: christian.gourieroux{at}ensae.fr
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Abstract |
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We derive necessary and sufficient conditions for the positive definiteness of the predicted volatility matrix in a bivariate autoregressive volatility specification. These nonlinear inequality restrictions have strong implications in terms of causality between volatilities and covolatilities.
KEYWORDS: GARCH model, nonlinear causality, stochastic volatility, Wishart process
We thanks F. Trojani for helpful discussion. The author gratefully
acknowledges financial support of NSERC Canada and of the Chair
AXA: "Large Risk in Insurance".
Received July 26, 2006;
revised April 17, 2007;
accepted April 18, 2007

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