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A124837
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Numerators of third-order harmonic numbers (defined by Conway and Guy, 1996).
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10
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1, 7, 47, 57, 459, 341, 3349, 3601, 42131, 44441, 605453, 631193, 655217, 1355479, 23763863, 24444543, 476698557, 162779395, 166474515, 34000335, 265842403, 812400067, 20666950267, 21010170067, 192066102203, 194934439103
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OFFSET
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1,2
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COMMENTS
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Denominators are A124838. All fractions reduced. Thanks to Jonathan Sondow for verifying these calculations. He suggests that the equivalent definition in terms of first order harmonic numbers may be computationally simpler. We are happy with the description of A027612 Numerator of 1/n + 2/(n-1) + 3/(n-2) + ... + (n-1)/2 + n, but baffled by the description of A027611.
a(n) is the numerator of H(n, (3)) = Sum_{m=1..n} Sum_{k=1..m} HarmonicNumber(k).
Denominators are listed in A124838.
p divides a(p-5) for prime p > 5.
Numbers k such that a(k) is prime are listed in A129881. (End)
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REFERENCES
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J. H. Conway and R. K. Guy, The Book of Numbers, New York: Springer-Verlag, pp. 143 and 258-259, 1996.
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LINKS
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Eric Weisstein's World of Mathematics, Harmonic Number. See equation for third order harmonic numbers.
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FORMULA
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a(n) = numerator(Sum_{m=1..n} Sum_{l=1..m} Sum_{k=1..l} 1/k).
a(n) = numerator(((n+2)!/(2!*n!)) * Sum_{k=3..n+2} 1/k).
a(n) = numerator(((n+2)*(n+1)/2) * Sum_{k=3..n+2} 1/k). (End)
a(n) = numerator(Sum_{k=0..n-1} (-1)^k*binomial(-3,k)/(n-k)). - Gary Detlefs, Jul 18 2011
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EXAMPLE
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a(1) = 1 = numerator of 1/1.
a(2) = 7 = numerator of 1/1 + 5/2 = 7/2.
a(3) = 47 = numerator of 7/2 + 13/3 = 47/6.
a(4) = 57 = numerator of 47/6 + 77/12 = 57/4.
a(5) = 549 = numerator of 57/4 + 87/10 = 549/20.
a(6) = 341 = numerator of 549/20 + 223/20 = 341/10
a(7) = 3349 = numerator of 341/10 + 481/35 = 3349/70.
a(8) = 88327 = numerator of 3349/70 + 4609/280 = 88327/1260.
a(9) = 3844 = numerator of 88327/1260 + 4861/252 = 3844/45.
a(10) = 54251 = numerator of 3844/45 + 55991/2520 = 54251/504, or, untelescoping:
a(10) = 54251 = numerator of 1/1 + 5/2 + 13/3 + 77/12 + 87/10 + 223/20 + 481/35 + 4609/252 + 4861/252 + 55991/2520 = 54251/504.
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MATHEMATICA
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Table[Numerator[(n+2)!/2!/n!*Sum[1/k, {k, 3, n+2}]], {n, 1, 30}] (* Alexander Adamchuk, Nov 11 2006 *)
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PROG
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(Haskell)
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CROSSREFS
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KEYWORD
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easy,frac,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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