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%I #41 Feb 27 2020 04:21:43
%S 15,1023,29127,355744082763
%N 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 (2,4)-perfect numbers.
%C See also the Cohen-te Riele links under A019276.
%C No other terms < 5*10^11. - _Jud McCranie_, Feb 08 2012
%C a(5) > 4*10^12, if it exists. - _Giovanni Resta_, Feb 26 2020
%H Graeme L. Cohen and Herman J. J. te Riele, <a href="http://projecteuclid.org/euclid.em/1047565640">Iterating the sum-of-divisors function</a>, Experimental Mathematics, 5 (1996), pp. 93-100.
%t Select[Range[100000], DivisorSigma[1, DivisorSigma[1, #]]/# == 4 &] (* _Robert Price_, Apr 07 2019 *)
%o (PARI) isok(n) = sigma(sigma(n))/n == 4; \\ _Michel Marcus_, May 12 2016
%Y Cf. A019278, A019279, A019281, A019283, A019284, A019285, A019286, A019287, A019288, A019289, A019290, A019291.
%K nonn,more
%O 1,1
%A _N. J. A. Sloane_
%E a(4) from _Jud McCranie_, Feb 08 2012
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