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A019290 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. 9
57120, 932064, 3932040, 251650560 (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

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

LINKS

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.

PROG

(PARI) isok(n) = sigma(sigma(n))/n == 13; \\ Michel Marcus, May 15 2016

CROSSREFS

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

KEYWORD

nonn,more

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified September 27 07:02 EDT 2021. Contains 347673 sequences. (Running on oeis4.)