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A049104
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Numbers k such that k and k-1 both have 6 divisors.
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4
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45, 76, 99, 117, 148, 172, 243, 244, 245, 333, 388, 508, 549, 604, 605, 725, 845, 909, 932, 964, 1076, 1084, 1252, 1325, 1413, 1468, 1557, 1588, 1676, 1773, 2524, 2525, 2637, 2645, 2764, 3284, 3357, 3412, 3509, 3789, 3988, 4076, 4204, 4205, 4419, 4492, 4805, 4869, 4924, 4925
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OFFSET
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1,1
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REFERENCES
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David Wells, The Penguin Dictionary of Curious and Interesting Numbers, Penguin Books, 1986, p. 103.
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LINKS
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FORMULA
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MATHEMATICA
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Flatten[Position[Partition[Table[DivisorSigma[0, n-1], {n, 5000}], 2, 1], _?(#=={6, 6} &)]] (* Vincenzo Librandi, Oct 21 2012 *)
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PROG
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(PARI) isok(n) = (numdiv(n) == 6) && (numdiv(n-1) == 6); \\ Michel Marcus, Feb 11 2016
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CROSSREFS
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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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