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 A378508 #8 Nov 30 2024 06:26:10
%S A378508 1,2,4,6,8,10,12,16,18,20,22,24,28,32,36,40,44,48,52,54,56,60,64,72,
%T A378508 80,82,84,88,92,96,100,104,108,112,120,128,130,132,144,156,160,162,
%U A378508 164,168,172,176,178,180,184,190,192,200,204,208,212,216,220,224,232,238,240,250,252,256,260,264,272,276,280,288,292,300,312,320,324,328,336,344,348,352,356,358,360,368,380,384,396,400
%N A378508 Values taken by phi(phi(m)) (A010554).
%C A378508 Numbers k such that A378506(k) > 0.
%H A378508 Amiram Eldar, <a href="/A378508/b378508.txt">Table of n, a(n) for n = 1..10000</a>
%H A378508 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%H A378508 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 A378508 q[k_] := AnyTrue[PhiInverse[k], PhiMultiplicity[#] > 0 &]; Select[Range[1000], q] (* using David M. Bressoud's CNT.m *)
%o A378508 (PARI) is(k) = {my(v = invphi(k)); for(i = 1, #v, if(istotient(v[i]), return(1))); 0;} \\ using _Max Alekseyev_'s invphi.gp
%Y A378508 Cf. A000010, A010554, A378506.
%Y A378508 Subsequence of A002202.
%K A378508 nonn
%O A378508 1,2
%A A378508 _Amiram Eldar_, Nov 29 2024