OFFSET
1,1
COMMENTS
a(1) and a(2) are 4-perfect numbers (A027687). a(4) = S(a(3)).
a(6), a(8), a(9), a(12), and a(15) are also 4-perfect numbers (A027687). - Michel Marcus, Feb 23 2014
LINKS
Eric Weisstein's World of Mathematics, Sociable Numbers
PROG
(PARI) isok(n) = (type(s= sigma(n)/4) == "t_INT") && (sigma(s)/4 == n); \\ Michel Marcus, Feb 20 2014
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Yasutoshi Kohmoto, Jan 27 2006
EXTENSIONS
a(5)-a(15) added by Michel Marcus, Feb 24 2014
STATUS
approved