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Formulation analytique<br />

Par symétrie :<br />

En multipliant <strong>par</strong> N :<br />

CN = N + 1 +<br />

N<br />

k=1<br />

CN = N + 1 + 2<br />

N<br />

NCN = N(N + 1) + 2<br />

1<br />

N (Ck−1 + CN−k)<br />

N<br />

k=1<br />

Ck−1<br />

N<br />

k=1<br />

En soustrayant la même formule pour N − 1 :<br />

En rassemblant les termes :<br />

Ck−1<br />

NCN − (N − 1)CN−1 = 2N + 2CN−1<br />

NCN = (N + 1)CN−1 + 2N<br />

301

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