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

1,1

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

Links

Table of n, a(n) for n=1..23.

Mathematica

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

Crossrefs

Cf. A028491.

Sequence in context: A095921 A106037 A136240 * A287142 A076220 A258213

Adjacent sequences: A090744 A090745 A090746 * A090748 A090749 A090750

Keyword

nonn,more,hard

Author

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

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

Status

approved