A107912 Numbers k that require two iterations of the sigma function to be >= 2*k.

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

Offset: 1

Author

Jud McCranie, May 27 2005

Keywords

• nonn

Examples

			sigma(8) = 15 but sigma(sigma(8)) = 25, so 8 is in the sequence.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000203, A055020, A055021, A107913, A107914.

Programs

Mathematica

ds2Q[n_]:=Module[{ds1=DivisorSigma[1,n]},ds1<2n<=DivisorSigma[1,ds1]]; Select[Range[120],ds2Q]  (* Harvey P. Dale, Feb 13 2011 *)

PARI

is(n) = {my(m = n, nn = 2*n, c = 0); while(m < nn, m = sigma(m); c++); c == 2;} \\ Amiram Eldar, Mar 18 2024

Extensions

Offset 1 by Michel Marcus, Apr 25 2020