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 A019286 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 (2,9)-perfect numbers. 2
 168, 10752, 331520, 691200, 1556480, 1612800, 106151936, 5099962368 (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 4010593484800 is also a term. See comment in A019278. - Michel Marcus, May 15 2016 LINKS Graeme L. Cohen and Herman J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 93-100. PROG (PARI) isok(n) = sigma(sigma(n))/n  == 9; \\ Michel Marcus, May 12 2016 CROSSREFS Cf. A000668, A019278, A019279, A019282, A019283, A019284. Sequence in context: A278010 A006363 A271033 * A218413 A064767 A115222 Adjacent sequences:  A019283 A019284 A019285 * A019287 A019288 A019289 KEYWORD nonn,more AUTHOR EXTENSIONS a(8) by Jud McCranie, Jan 28 2012 STATUS approved

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