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 A071386 #24 Apr 04 2024 08:09:57
%S A071386 2,8,20,32,40,44,48,56,60,72,92,96,104,108,116,120,128,132,140,144,
%T A071386 156,164,192,204,212,216,220,240,252,260,272,276,296,300,332,344,356,
%U A071386 360,368,380,384,392,396,400,416,420,440,444,452,456,476,480,500,504,512
%N A071386 Numbers k such that the cardinality of the set of solutions to phi(x) = k is odd.
%H A071386 Amiram Eldar, <a href="/A071386/b071386.txt">Table of n, a(n) for n = 1..10000</a>
%H A071386 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%F A071386 { k : Card(InvPhi(k)) mod 2 = 1 }.
%e A071386 k = 40 is a term: InvPhi(40) = {41,55,75,82,88,100,110,132,150} has 9 entries.
%o A071386 (PARI) is(n) = invphiNum(n) % 2; \\ _Amiram Eldar_, Mar 28 2024, using _Max Alekseyev_'s invphi.gp (see links).
%Y A071386 Cf. A000010 (phi), A014197, A058277, A063512, A071387.
%K A071386 nonn
%O A071386 1,1
%A A071386 _Labos Elemer_, May 23 2002