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THÈSE Estimation, validation et identification des modèles ARMA ...

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Chapitre 2. Estimating weak structural V<strong>ARMA</strong> models 78<br />

2.9 Appendix of additional example<br />

Example 2.2. Denoting by a0i(k,ℓ) and b0i(k,ℓ) the generic elements of the matrices<br />

A0i and B0i, the Kronecker indices are defined by pk = max{i : a0i(k,ℓ) =<br />

0 or b0i(k,ℓ) = 0 for some ℓ = 1,...,d}. To ensure relatively parsimonious param<strong>et</strong>erizations,<br />

one can specify an echelon form depending on the Kronecker indices(p1,...,pd).<br />

The reader is refereed to Lütkepohl (1993) for d<strong>et</strong>ails about the echelon form. For instance,<br />

a 3-variate <strong>ARMA</strong> process with Kronecker indices (1,2,0) admits the echelon<br />

form<br />

=<br />

⎛<br />

⎝<br />

⎛<br />

⎝<br />

1 0 0<br />

0 1 0<br />

× × 1<br />

1 0 0<br />

0 1 0<br />

× × 1<br />

⎞<br />

⎛<br />

⎠Xt −⎝<br />

⎞<br />

⎛<br />

⎠ǫt −⎝<br />

× × 0<br />

0 × 0<br />

0 0 0<br />

× × ×<br />

× × ×<br />

0 0 0<br />

⎞<br />

⎛<br />

⎠Xt−1 −⎝<br />

⎞<br />

⎛<br />

⎠ǫt−1 −⎝<br />

0 0 0<br />

× × 0<br />

0 0 0<br />

0 0 0<br />

× × ×<br />

0 0 0<br />

⎞<br />

⎠Xt−2<br />

⎞<br />

⎠ǫt−2<br />

where × denotes an unconstrained element. The variance of ǫt is defined by 6 additional<br />

param<strong>et</strong>ers. This echelon form thus corresponds to a param<strong>et</strong>rization by a vector ϑ of<br />

size k0 = 24.<br />

2.9.1 Verification of Assumption A8 on Example 2.1<br />

Thus<br />

In this example, we have<br />

<br />

Mϑ0 =<br />

α01 α02 σ 2 01 σ 2 01 α03<br />

α01α03 +α04 α02α03 +α05 σ 2 01 α03 σ 2 01 α2 03 +σ2 02<br />

<br />

Mϑ0=<br />

is of full rank k0 = 7.<br />

⎛<br />

⎜<br />

⎝<br />

1 α03 0 0 0 0 0 0<br />

0 0 1 α03 0 0 0 0<br />

0 α01 0 α02 0 σ 2 01 σ2 01 2α03σ 2 01<br />

0 1 0 0 0 0 0 0<br />

0 0 0 1 0 0 0 0<br />

0 0 0 0 1 α03 α03 α 2 03<br />

0 0 0 0 0 0 0 1<br />

⎞<br />

⎟<br />

⎠<br />

<br />

.

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