This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247111 #36 Nov 15 2024 03:05:43 %S A247111 6,28,36,496,8128,33550336,8589869056 %N A247111 Integers k such that sigma(sigma(k) - k) = 2*k, where sigma is the sum of divisors, A000203. %C A247111 That is, integers k such that A072869(k) = 2*k. %C A247111 All perfect numbers (A000396) belong to this sequence. %C A247111 Is there another term like 36 that is not perfect? %C A247111 a(8) > 10^11. - _Hiroaki Yamanouchi_, Sep 11 2015 %C A247111 a(8) <= 137438691328. - _David A. Corneth_, Jun 04 2021 %H A247111 <a href="/index/O#opnseqs">Index entries for sequences where odd perfect numbers must occur, if they exist at all</a> %e A247111 For k=36, sigma(sigma(36)-36) = sigma(91-36) = sigma(55) = 72, hence 36 is in the sequence. %t A247111 Select[Range[1,10000],DivisorSigma[1,DivisorSigma[1,#]-#]==2*#&] (* _Julien Kluge_, Sep 20 2016 *) %o A247111 (PARI) isok(n) = (sigma(sigma(n) - n) == 2*n); %Y A247111 Cf. A000203 (sigma(n)), A000396 (perfect numbers), A001065 (sigma(n)-n), A072869 (sigma(sigma(n)-n)). %Y A247111 Cf. also A019283, A326181, A342922. %K A247111 nonn,more %O A247111 1,1 %A A247111 _Michel Marcus_, Nov 19 2014 %E A247111 a(7) from _Michel Marcus_, Nov 22 2014