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 A072868 #42 Feb 18 2024 08:20:02
%S A072868 4,8,32,128,8192,131072,524288,2147483648,2305843009213693952,
%T A072868 618970019642690137449562112
%N A072868 Numbers k such that sigma(sigma(k) - k) = k.
%C A072868 Because numbers of the form 1+A000668(.) are a subsequence, 162259276829213363391578010288128 and 170141183460469231731687303715884105728 are also in the sequence (not necessarily the next ones). - _Joerg Arndt_, Jul 01 2012
%F A072868 a(n) = 1 + A000668(n) (conjecture), which implies equivalent conjectures stated in A000668.
%t A072868 f[n_]:=Plus@@Divisors[Plus@@Divisors[n]-n]==n; lst={};Do[If[f[n],AppendTo[lst,n]],{n,9!}];lst (* _Vladimir Joseph Stephan Orlovsky_, May 14 2010 *)
%t A072868 Select[Range[550000],DivisorSigma[1,DivisorSigma[1,#]-#]==#&] (* The program generates the first 7 terms of the sequence. Increasing the Range constant will generate more but it will run a long time. *) (* _Harvey P. Dale_, May 08 2018 *)
%o A072868 (PARI) isok(n) = sigma(sigma(n)-n) == n; \\ _Michel Marcus_, May 06 2016
%Y A072868 Cf. A000043.
%K A072868 nonn,more
%O A072868 1,1
%A A072868 _Benoit Cloitre_, Jul 27 2002
%E A072868 a(6) inserted by _Jaroslav Krizek_, Jun 29 2012