OFFSET
1,1
COMMENTS
See also the Cohen-te Riele links under A019276.
No other terms < 5*10^11. - Jud McCranie, Feb 08 2012
4010593484800 is also a term. See comment in A019278. - Michel Marcus, May 15 2016
LINKS
Graeme L. Cohen and Herman J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 93-100.
PROG
(PARI) isok(n) = sigma(sigma(n))/n == 9; \\ Michel Marcus, May 12 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
EXTENSIONS
a(8) by Jud McCranie, Jan 28 2012
a(9) from Giovanni Resta, Feb 26 2020
STATUS
approved