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 A071388 #16 Nov 15 2024 07:00:09
%S A071388 1,2,8,10,20,22,28,30,32,44,46,48,52,54,56,58,66,70,72,78,82,92,96,
%T A071388 102,104,106,110,116,120,126,130,132,136,138,140,148,150,156,164,166,
%U A071388 172,178,190,196,198,204,210,212,216,220,222,226,228,238,240,250,260,262
%N A071388 Numbers k such that the cardinality of the set of solutions to phi(x) = k is a prime.
%C A071388 All terms except 1 are even. - _Robert Israel_, Mar 29 2020
%H A071388 Robert Israel, <a href="/A071388/b071388.txt">Table of n, a(n) for n = 1..10000</a>
%H A071388 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%e A071388 InvPhi(48) = {65,104,105,112,130,140,144,156,168,180,210} has 11 terms, so 48 is a term.
%p A071388 filter:= n -> isprime(nops(numtheory:-invphi(n))):
%p A071388 select(filter, [$1..400]); # _Robert Israel_, Mar 29 2020
%o A071388 (PARI) is(k) = isprime(invphiNum(k)); \\ _Amiram Eldar_, Nov 15 2024, using _Max Alekseyev_'s invphi.gp
%Y A071388 Cf. A000010, A007366, A007367, A014197, A058277, A060668, A060670, A060674, A063512, A071386-A071389.
%K A071388 nonn
%O A071388 1,2
%A A071388 _Labos Elemer_, May 23 2002