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A069085 Abundant numbers n such that n = sigma(k) - 2k, where k = sigma(n) - 2n. 4
120, 672, 45840, 51168, 523776, 459818240, 1476304896, 51001180160 (list; graph; refs; listen; history; text; internal format)
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
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
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

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Last modified July 19 22:34 EDT 2024. Contains 374441 sequences. (Running on oeis4.)