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 A247111 Integers such that sigma(sigma(n) - n) = 2*n, where sigma is the sum of divisors, A000203. 1
 6, 28, 36, 496, 8128, 33550336, 8589869056 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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? a(8) > 10^11. - Hiroaki Yamanouchi, Sep 11 2015 LINKS EXAMPLE For n=36, sigma(sigma(36)-36) = sigma(91-36) = sigma(55) = 72, hence 36 is in the sequence. MATHEMATICA Select[Range[1, 10000], DivisorSigma[1, DivisorSigma[1, #]-#]==2*#&] (* Julien Kluge, Sep 20 2016 *) PROG (PARI) isok(n) = (sigma(sigma(n) - n) == 2*n); CROSSREFS Cf. A000203 (sigma(n)), A000396 (perfect numbers), A001065 (sigma(n)-n), A072869 (sigma(sigma(n)-n). Sequence in context: A117948 A252234 A242344 * A071834 A295078 A055196 Adjacent sequences:  A247108 A247109 A247110 * A247112 A247113 A247114 KEYWORD nonn,more AUTHOR Michel Marcus, Nov 19 2014 EXTENSIONS a(7) from Michel Marcus, Nov 22 2014 STATUS approved

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Last modified March 29 21:32 EDT 2020. Contains 333117 sequences. (Running on oeis4.)