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A124879
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Numbers k such that A027612(k) is prime.
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4
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2, 3, 6, 9, 18, 25, 29, 30, 39, 84, 91, 125, 130, 184, 195, 199, 203, 241, 245, 273, 281, 378, 552, 571, 653, 776, 901, 1099, 1215, 1224, 1235, 1315, 1412, 1657, 1942, 2076, 2085, 2743, 2745, 2855, 2859, 3517, 3717, 4183, 4188, 4362, 4547, 4728, 4783
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OFFSET
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1,1
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LINKS
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EXAMPLE
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A027612 begins {1, 5, 13, 77, 87, 223, 481, 4609, 4861, ...}.
Thus a(1) = 2, a(2) = 3, a(3) = 6, a(4) = 9.
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MATHEMATICA
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s=1; Do[s=s+1/(n+1); f=Numerator[(n+1)*(s-1)]; If[PrimeQ[f], Print[{n, f}]], {n, 1, 1942}]
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PROG
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(PARI) isok(n) = isprime(numerator(sum(k=1, n, k/(n-k+1)))); \\ Michel Marcus, Jul 14 2018
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CROSSREFS
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A027612(n) are the numerators of second order harmonic numbers H(n, (2)).
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KEYWORD
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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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