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A071834
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Numbers n > 1 such that n and sigma(n) have the same largest prime factor.
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3
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6, 28, 40, 84, 117, 120, 135, 140, 224, 234, 270, 420, 468, 496, 585, 672, 756, 775, 819, 891, 931, 936, 1080, 1120, 1170, 1287, 1372, 1488, 1550, 1625, 1638, 1782, 1862, 2176, 2299, 2325, 2340, 2480, 2574, 2793, 3100, 3159, 3250, 3276, 3360, 3472
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OFFSET
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1,1
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COMMENTS
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By pure convention, we could include a leading 1 to this sequence, as someone using the mathematically arguably value A006530(1) = 1 might search for this sequence with a leading 1. However, this was not done in view of the age of this sequence. - Rémy Sigrist, Jan 09 2018
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LINKS
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FORMULA
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EXAMPLE
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1550 = 2*5^2*31 and sigma(1550) = 2976 = 2^5*3*31 hence 1550 is in the sequence.
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MATHEMATICA
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fQ[n_] := FactorInteger[n][[-1, 1]] == FactorInteger[DivisorSigma[1, n]][[-1, 1]]; Rest@ Select[ Range@3500, fQ] (* Robert G. Wilson v, Jan 09 2018 *)
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PROG
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(PARI) for(n=2, 1000, if(component(component(factor(n), 1), omega(n)) == component(component(factor(sigma(n)), 1), omega(sigma(n))), print1(n, ", ")))
(PARI) isok(n) = vecmax(factor(n)[, 1]) == vecmax(factor(sigma(n))[, 1]); \\ Michel Marcus, Sep 29 2017
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CROSSREFS
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A000396 (perfect numbers) is a subsequence.
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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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