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A247111
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Integers such that sigma(sigma(n) - n) = 2*n, where sigma is the sum of divisors, A000203.
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1
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OFFSET
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1,1
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COMMENTS
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That is, integers such that A072869(n) = 2*n.
All perfect numbers (A000396) belong to this sequence.
Is there another term like 36 that is not perfect?
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LINKS
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EXAMPLE
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For n=36, sigma(sigma(36)-36) = sigma(91-36) = sigma(55) = 72, hence 36 is in the sequence.
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MATHEMATICA
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Select[Range[1, 10000], DivisorSigma[1, DivisorSigma[1, #]-#]==2*#&] (* Julien Kluge, Sep 20 2016 *)
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PROG
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(PARI) isok(n) = (sigma(sigma(n) - n) == 2*n);
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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