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A213785
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a(n) = Sum(P(i)*P(j), 1<=i<j<=n), where P(k) is the k-th Pell number A000129(k).
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2
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0, 0, 2, 17, 113, 693, 4123, 24234, 141738, 827298, 4824716, 28127435, 163955435, 955642695, 5569991317, 32464523892, 189217679988, 1102842830628, 6427842380918, 37464218883749, 218357488856453, 1272680757553593, 7417727160997231, 43233682460792670
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OFFSET
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0,3
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COMMENTS
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LINKS
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FORMULA
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G.f.: x^2*(2-x)/((1-x)*(1-6*x+x^2)*(1-2*x-x^2)). [Bruno Berselli, Jun 20 2012]
a(n) = 9*a(n-1) -20*a(n-2) +8*a(n-3) +5*a(n-4) -a(n-5). - Vincenzo Librandi, Jun 20 2012
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MATHEMATICA
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LinearRecurrence[{9, -20, 8, 5, -1}, {0, 0, 2, 17, 113}, 30] (* Vincenzo Librandi, Jun 20 2012 *)
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PROG
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(Magma) I:=[0, 0, 2, 17, 113]; [n le 5 select I[n] else 9*Self(n-1)-20*Self(n-2)+8*Self(n-3)+5*Self(n-4)-Self(n-5): n in [1..30]]; // Vincenzo Librandi, Jun 20 2012
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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