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A069169
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Nonprimes k such that sigma(k)*Sum_{d|k} 1/sigma(d) is an integer.
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1
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1, 6, 10, 14, 15, 18, 21, 22, 26, 28, 30, 33, 34, 35, 38, 39, 40, 42, 46, 51, 52, 55, 57, 58, 62, 65, 66, 69, 70, 74, 76, 77, 78, 82, 84, 85, 86, 87, 90, 91, 93, 94, 95, 102, 105, 106, 110, 111, 114, 115, 118, 119, 120, 122, 123, 124, 126, 129, 130, 133, 134, 138, 140
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OFFSET
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1,2
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LINKS
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MATHEMATICA
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q[n_] := IntegerQ[DivisorSigma[1, n]*DivisorSum[n, 1/DivisorSigma[1, #] &]]; Select[Range[140], !PrimeQ[#] && q[#] &] (* Amiram Eldar, Apr 30 2022 *)
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PROG
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(PARI) isok(k) = !isprime(k) && (denominator(sigma(k)*sumdiv(k, d, 1/sigma(d))) == 1); \\ Michel Marcus, Apr 30 2022
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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