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A019284
Let sigma_m (n) be result of applying sum-of-divisors function m times to n; call n (m,k)-perfect if sigma_m (n) = k*n; sequence gives the (2,7)-perfect numbers.
13
24, 1536, 47360, 343976, 572941926400
OFFSET
1,1
COMMENTS
See also the Cohen-te Riele links under A019276.
No other terms < 5*10^11. - Jud McCranie, Feb 08 2012
572941926400 is also a term. See comment in A019278. - Michel Marcus, May 15 2016
a(6) > 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[50000], DivisorSigma[1, DivisorSigma[1, #]]/# == 7 &] (* Robert Price, Apr 07 2019 *)
PROG
(PARI) isok(n) = sigma(sigma(n))/n == 7; \\ Michel Marcus, May 12 2016
KEYWORD
nonn,more
EXTENSIONS
a(5) from Giovanni Resta, Feb 26 2020
STATUS
approved