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A355990
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a(n) = n! / (6 * floor(n/3)!).
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5
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1, 4, 20, 60, 420, 3360, 10080, 100800, 1108800, 3326400, 43243200, 605404800, 1816214400, 29059430400, 494010316800, 1482030950400, 28158588057600, 563171761152000, 1689515283456000, 37169336236032000, 854894733428736000, 2564684200286208000
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OFFSET
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3,2
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LINKS
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FORMULA
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E.g.f.: (1 - x^3) * (exp(x^3) - 1)/(6 * (1 - x)).
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MATHEMATICA
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a[n_] := n!/(6 * Floor[n/3]!); Array[a, 22, 3] (* Amiram Eldar, Jul 22 2022 *)
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PROG
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(PARI) a(n) = n!/(6*(n\3)!);
(PARI) my(N=30, x='x+O('x^N)); Vec(serlaplace((1-x^3)*(exp(x^3)-1)/(6*(1-x))))
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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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