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A064592
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Nonunitary doubly perfect numbers: the sum of the nonunitary divisors of n is 2n; i.e., sigma(n) - usigma(n) = 2n.
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6
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2520, 31320, 1163160, 2208384, 3053232, 6535296, 13472928, 123165600, 365959296, 4401782352, 5517818880, 612014161920
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OFFSET
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1,1
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COMMENTS
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There are no others up to 7.9*10^12, but here's a larger one: 2846972800010800140288.
Ligh & Wall found the first 11 terms of this sequence. - Amiram Eldar, Sep 27 2018
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LINKS
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MATHEMATICA
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nusigma[ n_ ] := DivisorSigma[ 1, n ]-Times@@(1+Power@@#&/@FactorInteger[ n ]); For[ n=1, True, n++, If[ nusigma[ n ]==2n, Print[ n ] ] ]
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PROG
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(PARI) isok(n) = sumdiv(n, d, if (gcd(d, n/d)!=1, d)) == 2*n; \\ Michel Marcus, Jul 31 2017
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CROSSREFS
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KEYWORD
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nonn,hard
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AUTHOR
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STATUS
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approved
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