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A027759
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Numerator of Sum_{p prime, p-1|n} 1/p.
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3
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1, 5, 1, 31, 1, 41, 1, 31, 1, 61, 1, 3421, 1, 5, 1, 557, 1, 821, 1, 371, 1, 121, 1, 3421, 1, 5, 1, 929, 1, 15745, 1, 557, 1, 5, 1, 2557843, 1, 5, 1, 15541, 1, 1805, 1, 743, 1, 241, 1, 60887, 1, 61, 1, 1673, 1, 821, 1, 929, 1, 301, 1, 79085411, 1, 5, 1, 557
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OFFSET
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1,2
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LINKS
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FORMULA
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EXAMPLE
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1/2, 5/6, 1/2, 31/30, 1/2, 41/42, 1/2, 31/30, 1/2, 61/66, 1/2, 3421/2730, 1/2, 5/6, 1/2, 557/510, ...
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MATHEMATICA
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a[n_] := 1/(Select[Divisors[n], PrimeQ[# + 1]&] + 1) // Total // Numerator;
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PROG
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(PARI) a(n) = numerator(sumdiv(n, d, if (isprime(d+1), 1/(d+1)))); \\ Michel Marcus, May 06 2021
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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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