This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A107912 #20 Mar 18 2024 03:28:18
%S A107912 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,
%T A107912 39,41,44,45,46,47,50,51,52,53,55,57,58,59,62,63,64,65,68,69,71,74,75,
%U A107912 76,77,79,82,83,85,86,87,89,91,92,93,94,95,98,99,101,103,105,106,107
%N A107912 Numbers k that require two iterations of the sigma function to be >= 2*k.
%H A107912 Amiram Eldar, <a href="/A107912/b107912.txt">Table of n, a(n) for n = 1..10000</a>
%e A107912 sigma(8) = 15 but sigma(sigma(8)) = 25, so 8 is in the sequence.
%t A107912 ds2Q[n_]:=Module[{ds1=DivisorSigma[1,n]},ds1<2n<=DivisorSigma[1,ds1]]; Select[Range[120],ds2Q] (* _Harvey P. Dale_, Feb 13 2011 *)
%o A107912 (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
%Y A107912 Cf. A000203, A055020, A055021, A107913, A107914.
%K A107912 nonn
%O A107912 1,1
%A A107912 _Jud McCranie_, May 27 2005
%E A107912 Offset 1 by _Michel Marcus_, Apr 25 2020