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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. 11
 15, 1023, 29127, 355744082763 (list; graph; refs; listen; history; text; internal format)
 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 Cf. A019278, A019279, A019281, A019283, A019284, A019285, A019286, A019287, A019288, A019289, A019290, A019291. Sequence in context: A229840 A102102 A196569 * A082158 A064625 A241331 Adjacent sequences:  A019279 A019280 A019281 * A019283 A019284 A019285 KEYWORD nonn,more AUTHOR EXTENSIONS a(4) from Jud McCranie, Feb 08 2012 STATUS approved

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Last modified October 21 23:09 EDT 2020. Contains 337945 sequences. (Running on oeis4.)