|
| |
|
|
A107912
|
|
Numbers n that require two iterations of the sigma function to be >= 2n.
|
|
3
| |
|
|
2, 3, 4, 5, 7, 8, 10, 11, 14, 15, 16, 17, 19, 21, 22, 23, 26, 27, 29, 31, 32, 33, 34, 35, 38, 39, 41, 44, 45, 46, 47, 50, 51, 52, 53, 55, 57, 58, 59, 62, 63, 64, 65, 68, 69, 71, 74, 75, 76, 77, 79, 82, 83, 85, 86, 87, 89, 91, 92, 93, 94, 95, 98, 99, 101, 103, 105, 106, 107
(list; graph; refs; listen; history; internal format)
|
|
|
|
OFFSET
| 0,1
|
|
|
REFERENCES
| Cf. A000203, A055021, A107913, A107914.
|
|
|
EXAMPLE
| Sigma(8)=15 but sigma(sigma(8))=25, so 8 is in the sequence.
|
|
|
MATHEMATICA
| ds2Q[n_]:=Module[{ds1=DivisorSigma[1, n]}, ds1<2n<=DivisorSigma[1, ds1]]; Select[Range[120], ds2Q] (* From Harvey P. Dale, Feb 13 2011 *)
|
|
|
CROSSREFS
| Sequence in context: A025199 A180968 A191847 * A190850 A066077 A103740
Adjacent sequences: A107909 A107910 A107911 * A107913 A107914 A107915
|
|
|
KEYWORD
| nonn
|
|
|
AUTHOR
| Jud McCranie (JudMcCranie(AT)ugaalum.uga.edu), May 27 2005
|
| |
|
|
