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 A019282 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. 7

%I

%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 No other terms < 5 * 10^11. - _Jud McCranie_, Feb 08 2012

%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, 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.

%K nonn,more

%O 1,1

%A _N. J. A. Sloane_

%E a(4) from _Jud McCranie_, Feb 08 2012

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Last modified November 12 09:29 EST 2019. Contains 329054 sequences. (Running on oeis4.)