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.

168, 10752, 331520, 691200, 1556480, 1612800, 106151936, 5099962368, 4010593484800

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

Table of n, a(n) for n=1..9.

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, A019281, A019282, A019283, A019284, A019285, A019286, A019287, A019288, A019289, A019290, A019291.

Sequence in context: A278010 A006363 A271033 * A218413 A064767 A115222

Adjacent sequences: A019283 A019284 A019285 * A019287 A019288 A019289

Keyword

nonn,more

Author

N. J. A. Sloane

Extensions

a(8) by Jud McCranie, Jan 28 2012

a(9) from Giovanni Resta, Feb 26 2020

Status

approved