A113546 - OEIS

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

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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

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).

KEYWORD

nonn,more

AUTHOR

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