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
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