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1, 140, 270, 672, 1638, 2970, 6200, 8190, 18600, 18620, 27846, 30240, 32760, 55860, 105664, 117800, 167400, 173600, 237510, 242060, 332640, 360360, 539400, 695520, 726180, 753480, 950976, 1089270, 1421280, 1539720
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internal format)
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OFFSET
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1,2
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REFERENCES
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R. K. Guy, Unsolved Problems in Number Theory, B2.
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LINKS
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EXAMPLE
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A001599(4) = 140, but 336 = sigma(140) <> 2*140 = 280. Thus, 140 is a harmonic number which is not perfect. - Muniru A Asiru, Nov 26 2018
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MATHEMATICA
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Select[Range[2 10^7], IntegerQ[HarmonicMean[Divisors[#]]] && !DivisorSigma[1, #]==2 # &] (* Vincenzo Librandi, Nov 27 2018 *)
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PROG
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(GAP) Concatenation([1], Filtered([2, 4..2000000], n->Sigma(n)<>2*n and IsInt(n*Tau(n)/Sigma(n)))); # Muniru A Asiru, Nov 26 2018
(PARI) isok(n) = my(sn = sigma(n)); (frac(n*numdiv(n)/sn) == 0) && (sn != 2*n); \\ Michel Marcus, Nov 28 2018
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CROSSREFS
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For the associated harmonic means, see A102408.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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