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 A162923 #16 Dec 26 2024 03:56:07
%S A162923 2,3,5,6,9,10,11,13,16,17,18,19,20,21,22,23,25,26,27,29,30,31,33,34,
%T A162923 35,36,37,38,40,41,43,44,45,46,47,48,49,50,51,52,53,54,55,57,58,59,61,
%U A162923 62,64,65,66,67,68,69,70,71,73,74,75,76,77,78,79,81,82,83,85,86,87,88,89,92,93
%N A162923 Numbers k such that the equation sigma(sigma(x)) = k has no solutions.
%H A162923 Ivan Neretin, <a href="/A162923/b162923.txt">Table of n, a(n) for n = 1..10000</a>
%H A162923 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%F A162923 {A000027} minus {A070286}.
%t A162923 mx = 100; Complement[Range[mx], Table[Nest[DivisorSigma[1, #] &, n, 2], {n, mx}]] (* _Ivan Neretin_, Feb 12 2018 *)
%o A162923 (PARI) is(k) = {my(v = invsigma(k)); for(i = 1, #v, if(invsigmaNum(v[i]) > 0, return(0))); 1;} \\ _Amiram Eldar_, Dec 26 2024, using _Max Alekseyev_'s invphi.gp
%Y A162923 Complement of A070286.
%Y A162923 Cf. A051027.
%K A162923 nonn
%O A162923 1,1
%A A162923 _Jaroslav Krizek_, Jul 17 2009