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A128045
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a(n) = denominator of b(n), where b(1) = 1, b(n) = Sum_{k=1..n-1} b(n-k) * H(k); H(k) = Sum_{j=1..k} 1/j, the k-th harmonic number.
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2
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1, 1, 2, 6, 2, 5, 360, 2520, 1680, 15120, 2700, 11880, 9979200, 8648640, 18345600, 2476656000, 27243216000, 714714000, 427508928000, 1160381376000, 1055947052160000, 22174888095360000, 38718058579200, 141031842336000
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OFFSET
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1,3
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LINKS
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FORMULA
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G.f. for fractions: x / (1 + log(1 - x) / (1 - x)). - Ilya Gutkovskiy, Sep 01 2021
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EXAMPLE
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1, 1, 5/2, 35/6, 27/2, 156/5, 25951/360, 419681/2520, 646379/1680, 13439609/15120, 5544403/2700, 56359019/11880, ...
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MATHEMATICA
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f[l_List] := Block[{n = Length[l] + 1}, Append[l, Sum[l[[n - k]]*HarmonicNumber[k], {k, n - 1}]]]; Denominator[Nest[f, {1}, 24]] (* Ray Chandler, Feb 12 2007 *)
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CROSSREFS
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KEYWORD
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nonn,frac
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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