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A069168
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Numbers k such that sigma(k) * Sum_{d|k} (1/sigma(d)) is not an integer.
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1
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4, 8, 9, 12, 16, 20, 24, 25, 27, 32, 36, 44, 45, 48, 49, 50, 54, 56, 60, 63, 64, 68, 72, 75, 80, 81, 88, 92, 96, 98, 99, 100, 104, 108, 112, 116, 117, 121, 125, 128, 132, 135, 136, 144, 147, 150, 153, 160, 162, 164, 168, 169, 171, 175, 176, 180, 184, 188, 189, 192
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OFFSET
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1,1
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COMMENTS
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LINKS
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MATHEMATICA
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fQ[n_] := !IntegerQ[ DivisorSigma[1, n] Plus @@ (1/DivisorSigma[1, Divisors[n]])]; Select[ Range@ 200, fQ] (* Robert G. Wilson v, Jul 21 2012 *)
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PROG
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(PARI) isok(k) = denominator(sigma(k)*sumdiv(k, d, 1/sigma(d))) != 1; \\ Michel Marcus, Feb 09 2021
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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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