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A019290
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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 (2,13)-perfect numbers.
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9
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OFFSET
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1,1
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COMMENTS
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See also the Cohen-te Riele links under A019276.
No other terms < 5*10^11. - Jud McCranie, Feb 08 2012
11383810648416 is also a term. See comment in A019278. - Michel Marcus, May 15 2016
a(5) > 4*10^12. - Giovanni Resta, Feb 26 2020
50248050278400, 117245450649600, 86575337046016000 are also terms. - Michel Marcus, Feb 27 2020
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LINKS
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Table of n, a(n) for n=1..4.
Graeme L. Cohen and Herman J. J. te Riele, Iterating the sum-of-divisors function, Experimental Mathematics, 5 (1996), pp. 93-100.
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PROG
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(PARI) isok(n) = sigma(sigma(n))/n == 13; \\ Michel Marcus, May 15 2016
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CROSSREFS
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Cf. A019276, A019278, A019279, A019281, A019282, A019283, A019284, A019285, A019286, A019287, A019288, A019289, A019291.
Sequence in context: A292788 A138597 A206758 * A069335 A236152 A340434
Adjacent sequences: A019287 A019288 A019289 * A019291 A019292 A019293
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KEYWORD
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nonn,more
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AUTHOR
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N. J. A. Sloane.
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STATUS
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approved
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