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Processus de Lévy en Finance - Laboratoire de Probabilités et ...

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94 CHAPTER 2. THE CALIBRATION PROBLEM<br />

Choose a subsequ<strong>en</strong>ce of {Q δ k}, converging weakly to Q ∗ ∈ M ∩ L + B<br />

. To simplify notation,<br />

this subsequ<strong>en</strong>ce is <strong>de</strong>noted again by {Q δ k} k≥1 . Substituting r = δ into Equation (2.34) and<br />

making k t<strong>en</strong>d to infinity shows that<br />

Tog<strong>et</strong>her with Lemma 2.2 this implies that<br />

lim sup ‖C Qδk − C M ‖ 2 ≤ ‖C Q+ − C M ‖ 2 .<br />

k<br />

‖C Q∗ − C M ‖ 2 ≤ ‖C Q+ − C M ‖ 2 ,<br />

h<strong>en</strong>ce Q ∗ is a least squares solution. By weak lower semicontinuity of I (cf. Lemma 2.11) and<br />

using (2.36),<br />

I(Q ∗ |P ) ≤ lim inf<br />

k<br />

I(Q δ k<br />

|P ) ≤ lim sup I(Q δ k<br />

|P ) ≤ I(Q + |P ),<br />

k<br />

which means that Q ∗ is a MELSS. The last assertion of the theorem follows from the fact that<br />

in this case every subsequ<strong>en</strong>ce of {Q δ k} has a further subsequ<strong>en</strong>ce converging toward Q + .

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