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A307041
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a(n) = Sum_{k=0..floor(n/7)} (-1)^k*binomial(n,7*k).
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2
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1, 1, 1, 1, 1, 1, 1, 0, -7, -35, -119, -329, -791, -1715, -3430, -6419, -11319, -18767, -28763, -38759, -38759, 0, 149205, 571781, 1613129, 3964051, 8934121, 18874261, 37748522, 71705865, 129080161, 218205281, 339081225, 459957169, 459957169, 0, -1749692735
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OFFSET
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0,9
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LINKS
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FORMULA
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G.f.: (1 - x)^6/((1 - x)^7 + x^7).
a(n) = 7*a(n-1) - 21*a(n-2) + 35*a(n-3) - 35*a(n-4) + 21*a(n-5) - 7*a(n-6) for n > 6.
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MATHEMATICA
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a[n_] := Sum[(-1)^k * Binomial[n, 7*k], {k, 0, Floor[n/7]}]; Array[a, 37, 0] (* Amiram Eldar, May 25 2021 *)
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PROG
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(PARI) {a(n) = sum(k=0, n\7, (-1)^k*binomial(n, 7*k))}
(PARI) N=66; x='x+O('x^N); Vec((1-x)^6/((1-x)^7+x^7))
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CROSSREFS
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KEYWORD
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sign,easy
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AUTHOR
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STATUS
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approved
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