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