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 A090747 #25 Aug 25 2024 18:45:38 %S A090747 2,6,12,70,102,540,1090,1366,1626,4176,9010,9550,36912,43062,49680, %T A090747 57916,483610,877842,2215302,2704980,3598866,7973130,8530116 %N A090747 a(n) = A028491(n) - 1. %C A090747 If m is in the sequence then x=3^m is a solution for the equation sigma(sigma(x)) + sigma(x) = 3x. Does this equation have other solutions? - _Farideh Firoozbakht_, Jun 14 2010 %C A090747 Also numbers k such that sigma(3^k) is prime. - _Metin Sariyar_, Nov 18 2019 %t A090747 Select[Range[10000], PrimeQ[DivisorSigma[1, 3^#]] &] (* _Vincenzo Librandi_, Mar 26 2015 *) %Y A090747 Cf. A028491. %K A090747 nonn,more,hard %O A090747 1,1 %A A090747 mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004 %E A090747 More terms from _Vincenzo Librandi_, Mar 26 2015 %E A090747 a(19) derived from A028491 added by _Michel Marcus_, Nov 18 2019 %E A090747 a(20)-a(23) derived from A028491 added by _Jason Yuen_, Aug 25 2024