OFFSET
1,1
COMMENTS
See also the Cohen-te Riele links under A019276.
No other terms < 5*10^11. - Jud McCranie, Feb 08 2012
a(5) > 4*10^12, if it exists. - Giovanni Resta, Feb 26 2020
LINKS
Graeme L. Cohen and Herman J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 93-100.
MATHEMATICA
Select[Range[100000], DivisorSigma[1, DivisorSigma[1, #]]/# == 4 &] (* Robert Price, Apr 07 2019 *)
PROG
(PARI) isok(n) = sigma(sigma(n))/n == 4; \\ Michel Marcus, May 12 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
EXTENSIONS
a(4) from Jud McCranie, Feb 08 2012
STATUS
approved