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A074247
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Numbers n such that harmonic mean of the divisors of n is a prime.
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1
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6, 28, 140, 496, 2970, 8128, 27846, 105664, 117800, 237510, 539400, 1421280, 1539720, 2290260, 4358600, 8872200, 33550336, 50401728, 90409410, 233103780, 287425800, 318177800, 423184320, 559903400, 825120800, 1632825792
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Subsequence of A001599. Harmonic mean of divisors = n * tau(n) / sigma(n).
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EXAMPLE
| 140*tau(140)/sigma(140) = 5, a prime, so 140 is a term of the sequence.
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MATHEMATICA
| Select[Range[10^5], PrimeQ[ # * DivisorSigma[0, # ] / DivisorSigma[1, # ]] &]
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CROSSREFS
| Cf. A001599.
Sequence in context: A108051 A199315 A001599 * A053783 A110047 A163029
Adjacent sequences: A074244 A074245 A074246 * A074248 A074249 A074250
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KEYWORD
| nonn
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AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Sep 20 2002
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EXTENSIONS
| a(8)-a(26) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Jun 10 2010
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