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A368840
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a(n) = n! * (n+1)! * (n+2)! * Sum_{k=0..n} 1/(6^(n-k) * k! * (k+1)! * (k+2)!).
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1
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1, 2, 9, 91, 1821, 63736, 3569217, 299814229, 35977707481, 5936321734366, 1305990781560521, 373513363526309007, 135958864323576478549, 61861283267227297739796, 34642318629647286734285761, 23556776668160154979314317481
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = binomial(n+2,3)*a(n-1) + 1.
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PROG
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(PARI) a(n) = n!*(n+1)!*(n+2)!*sum(k=0, n, 1/(6^(n-k)*k!*(k+1)!*(k+2)!));
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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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