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 A378509 #8 Nov 30 2024 06:26:06
%S A378509 30,42,46,58,66,70,78,102,106,110,116,126,136,138,140,148,150,166,196,
%T A378509 198,210,222,226,228,262,268,270,282,294,296,306,310,316,330,332,342,
%U A378509 346,366,372,378,382,388,392,438,444,452,456,460,462,466,478,498,502,506
%N A378509 Totient numbers k for which there is no solution to the equation phi(phi(x)) = k.
%H A378509 Amiram Eldar, <a href="/A378509/b378509.txt">Table of n, a(n) for n = 1..10000</a>
%H A378509 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%H A378509 David M. Bressoud, <a href="https://www.davidbressoud.org/books-videos">A Course in Computational Number Theory (web page)</a>, <a href="https://drive.google.com/file/d/1UMesJ93mpdCabQvETMqOBH8eCR4JWA6X/view?usp=sharing">CNT.m</a>, Computational Number Theory Mathematica package.
%t A378509 q[k_] := Module[{v = PhiInverse[k]}, Length[v] > 0 && AllTrue[v, PhiMultiplicity[#] == 0 &]]; Select[Range[1000], q] (* using David M. Bressoud's CNT.m *)
%o A378509 (PARI) is(k) = {my(v = invphi(k)); if(#v == 0, return(0)); for(i = 1, #v, if(istotient(v[i]), return(0))); 1;} \\ using _Max Alekseyev_'s invphi.gp
%Y A378509 Intersection of A002202 (totient numbers) and the complement of A378508.
%Y A378509 Cf. A000010, A010554, A378510.
%K A378509 nonn
%O A378509 1,1
%A A378509 _Amiram Eldar_, Nov 29 2024