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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

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.

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

N. J. A. Sloane

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.)