A113546 Let S(n)=sigma(n)/3. Numbers k such that S(S(k))=k, 1/3-sociable number of order 1 or 2.

120, 672, 7680, 8184, 523776, 1100190, 1124352, 14913024, 16149760, 459818240, 775898880, 874897408, 1476304896, 51001180160

Offset

1,1

Comments

a(1), a(2), a(5) and a(10) are 3-perfect numbers (A005820). [Corrected by Mauro Fiorentini, Jul 21 2015]

a(4) = S(a(3)).

a(15) > 1.5*10^11. - Giovanni Resta, Jul 21 2015

Links

Table of n, a(n) for n=1..14.

Eric Weisstein's World of Mathematics, Sociable Numbers

Prog

(PARI) isok(n) = (type(s=sigma(n)/3) == "t_INT") && (sigma(s)/3 == n); \\ Michel Marcus, Feb 20 2014

Crossrefs

A005820 is a subsequence.

Cf. A113286 (1/4-sociable numbers of order 1 or 2).

Sequence in context: A292365 A306373 A356548 * A281771 A166069 A114887

Adjacent sequences: A113543 A113544 A113545 * A113547 A113548 A113549

Keyword

nonn,more

Author

Yasutoshi Kohmoto, Jan 27 2006

Extensions

a(6)-a(9) from Michel Marcus, Feb 20 2014

a(10)-a(12) from Michel Marcus, Feb 24 2014

a(13)-a(14) from Giovanni Resta, Jul 22 2015

Status

approved