A069085 Abundant numbers n such that n = sigma(k) - 2k, where k = sigma(n) - 2n.

120, 672, 45840, 51168, 523776, 459818240, 1476304896, 51001180160

Offset

1,1

Comments

All triply perfect numbers (A005820) are in the sequence, as are 45840 and 51168. Are there any other terms?

a(8) > 10^10. - Donovan Johnson, Apr 22 2012

a(9) > 1.3*10^11. - Giovanni Resta, Jun 06 2016

Links

Table of n, a(n) for n=1..8.

Example

sigma(45840) - 2*45840 = 51168 and sigma(51168) - 2*51168 = 45840, so 45840 and 51168 are in the sequence.

Mathematica

For[n=1, True, n++, k=DivisorSigma[1, n]-2n; If[k>0&&DivisorSigma[1, k]-2k==n, Print[n]]]

Crossrefs

Cf. A005820.

Sequence in context: A281771 A166069 A114887 * A039688 A005820 A306602

Adjacent sequences: A069082 A069083 A069084 * A069086 A069087 A069088

Keyword

more,nonn

Author

Naohiro Nomoto, Apr 05 2002

Extensions

Edited by Dean Hickerson, Apr 11 2002

a(7) from Donovan Johnson, Apr 22 2012

a(8) from Giovanni Resta, Jun 06 2016

Status

approved