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Search: id:A074245
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| A074245 |
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Numbers n such that sigma(n) is a harmonic number. |
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+0
1
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| 1, 5, 12, 76, 136, 139, 178, 269, 276, 308, 427, 429, 446, 455, 501, 581, 611, 612, 738, 932, 1576, 1637, 2952, 2969, 3184, 3204, 4647, 4975, 5400, 5458, 6199, 7152, 8816, 9120, 9180, 9196, 9272, 9294, 9504, 9584, 9720, 9950, 9960
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Recall that n is harmonic if the harmonic mean of its divisors is an integer, i.e. if n * tau(n) / sigma(n) is an integer. (Tattersall, p. 147)
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REFERENCES
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Tattersall, J. "Elementary Number Theory in Nine Chapters", Cambridge Univ. Press, 2001.
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EXAMPLE
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sigma(12) = 28 and 28 * tau(28) / sigma(28) = 28 * 6 / 56 = 3, an integer, so 12 is a term of the sequence.
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MATHEMATICA
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isHarmonic[n_] := IntegerQ[n*DivisorSigma[0, n] / DivisorSigma[1, n]]; Select[Range[10^4], isHarmonic[DivisorSigma[1, # ]] &]
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CROSSREFS
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Cf. A001599.
Sequence in context: A009429 A156236 A128323 this_sequence A064371 A009414 A009426
Adjacent sequences: A074242 A074243 A074244 this_sequence A074246 A074247 A074248
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KEYWORD
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nonn
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AUTHOR
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Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Sep 19 2002
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