A090747 a(n) = A028491(n) - 1.

2, 6, 12, 70, 102, 540, 1090, 1366, 1626, 4176, 9010, 9550, 36912, 43062, 49680, 57916, 483610, 877842, 2215302, 2704980, 3598866, 7973130, 8530116

Offset: 1

Author

mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004

Keywords

• nonn
• more
• hard

Comments

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

Also numbers k such that sigma(3^k) is prime. - Metin Sariyar, Nov 18 2019

Crossrefs

Cf. A028491.

Programs

Mathematica

Select[Range[10000], PrimeQ[DivisorSigma[1, 3^#]] &] (* Vincenzo Librandi, Mar 26 2015 *)

Extensions

More terms from Vincenzo Librandi, Mar 26 2015

a(19) derived from A028491 added by Michel Marcus, Nov 18 2019

a(20)-a(23) derived from A028491 added by Jason Yuen, Aug 25 2024