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 A107913 #19 Mar 18 2024 03:27:43
%S A107913 9,13,25,37,43,49,61,67,73,97,109,121,151,157,163,169,181,193,211,225,
%T A107913 229,241,277,283,289,313,331,337,361,373,397,409,421,433,457,487,523,
%U A107913 529,541,547,577,601,613,625,631,661,673,691,709,729,733,751,757,787
%N A107913 Numbers k that require three iterations of the sigma function to be >= 2*k.
%H A107913 Amiram Eldar, <a href="/A107913/b107913.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harvey P. Dale)
%e A107913 sigma(sigma(9)) = 14 but sigma(sigma(sigma(9))) = 24, so 9 is in the sequence.
%t A107913 is3Q[n_]:=Boole[#>=2n&/@NestList[DivisorSigma[1,#]&,n,3]]=={0,0,0,1}; Select[Range[ 800],is3Q] (* _Harvey P. Dale_, Jul 09 2023 *)
%o A107913 (PARI) is(n) = {my(m = n, nn = 2*n, c = 0); while(m < nn, m = sigma(m); c++); c == 3;} \\ _Amiram Eldar_, Mar 18 2024
%Y A107913 Cf. A000203, A055020, A055021, A107912, A107914.
%K A107913 nonn
%O A107913 1,1
%A A107913 _Jud McCranie_, May 27 2005
%E A107913 Offset 1 by _Michel Marcus_, Apr 25 2020