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A111918
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Numerator of x(n) = Sum_{k=1..n} ((odd part of k)/(k^3)).
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6
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1, 9, 89, 721, 18601, 2089, 103961, 832913, 68093153, 68347169, 8320810649, 8331482849, 1414167788681, 1416817979081, 1421435199689, 11373510649537, 3295255574810593, 366551352989977, 132591913780524097, 132652127531625601
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OFFSET
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1,2
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COMMENTS
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x(n) = a(n)/A111919(n) ---> Pi*Pi/7 = 6*zeta(2)/7.
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REFERENCES
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G. Pólya and G. Szegő, Problems and Theorems in Analysis II (Springer 1924, reprinted 1972), Part Eight, Chap. 1, Sect. 6, Problem 50.
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LINKS
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Eric Weisstein's World of Mathematics, Odd Part
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EXAMPLE
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a(50) = 429245027972423430658635002176171233144054521,
A111919(50) = 307330458857514095936081844184308729630720000:
x(50) = a(50)/A111919(50) = 1.39668..., x(50)*7/6 = 1.62946....
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MAPLE
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S:= 0: Res:= NULL:
for k from 1 to 25 do
S:= S + 1/k^2/2^padic:-ordp(k, 2);
Res:= Res, numer(S)
od:
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MATHEMATICA
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oddPart[n_] := n/2^IntegerExponent[n, 2];
x[n_] := Sum[oddPart[k]/k^3, {k, 1, n}];
a[n_] := Numerator[x[n]];
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PROG
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(Magma) val:=func<n|n/2^Valuation(n, 2)>; [Numerator(&+[val(k)/(k^3):k in [1..n]]):n in [1..20]]; // Marius A. Burtea, Jan 13 2020
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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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STATUS
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approved
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