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 A292369 #13 Sep 25 2017 06:00:32 %S A292369 2,4,16,25,64,4096,65536,262144,1073741824 %N A292369 Numbers n such that f(f(f(n))) = f(f(n)) + f(n) where f = A000203. %C A292369 Numbers n such that A066971(n) = A051027(n) + A000203(n). %C A292369 A061652 is a subsequence. %C A292369 Are there any odd terms other than 25? %e A292369 25 = 5^2 is a term because sigma(sigma(sigma(5^2))) = sigma(2^5) = sigma(sigma(5^2)) + sigma(5^2). %t A292369 f[n_] := DivisorSigma[1, n]; fQ[n_] := f[f[f[n]]] == f[f[n]] + f[n]; Select[ Range@1000000, fQ] (* _Robert G. Wilson v_, Sep 23 2017 *) %o A292369 (PARI) s(n) = sigma(n); %o A292369 isok(n) = s(s(s(n)))==s(s(n))+s(n); %Y A292369 Cf. A000203, A019279, A051027, A061652, A066971. %K A292369 nonn,more %O A292369 1,1 %A A292369 _Altug Alkan_, Sep 15 2017 %E A292369 a(9) from _Giovanni Resta_, Sep 15 2017