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A113546 Let S(n)=sigma(n)/3. Numbers k such that S(S(k))=k, 1/3-sociable number of order 1 or 2. 3
120, 672, 7680, 8184, 523776, 1100190, 1124352, 14913024, 16149760, 459818240, 775898880, 874897408, 1476304896, 51001180160 (list; graph; refs; listen; history; text; internal format)
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).
Sequence in context: A292365 A306373 A356548 * A281771 A166069 A114887
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

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Last modified July 20 06:55 EDT 2024. Contains 374441 sequences. (Running on oeis4.)