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 A060667 #15 Nov 17 2024 07:18:37
%S A060667 4,6,18,42,100,162,184,208,328,424,460,468,486,492,616,636,664,688,
%T A060667 700,712,784,820,900,904,1020,1060,1072,1168,1240,1264,1276,1288,1300,
%U A060667 1356,1360,1384,1404,1458,1480,1528,1672,1740,1768,1864,1896,1900,1908,2008
%N A060667 Numbers k such that phi(x) = k has exactly 4 solutions.
%H A060667 Amiram Eldar, <a href="/A060667/b060667.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe)
%H A060667 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%e A060667 18 = phi(19) = phi(27) = phi(38) = phi(54).
%t A060667 a = Table[ 0, {2500} ]; Do[ p = EulerPhi[ n ]; If[ p < 2501, a[ [ p ] ]++ ], {n, 1, 5000} ]; Select[ Range[ 2500 ], a[ [ # ] ] == 4 & ]
%o A060667 (PARI) is(k) = invphiNum(k) == 4 \\ _Amiram Eldar_, Nov 17 2024, using _Max Alekseyev_'s invphi.gp
%Y A060667 Cf. A000010.
%Y A060667 Number of solutions: A007617 (0), A007366 (2), A007367 (3), this sequence (4), A060668 (5), A060669 (6), A060670 (7), A060671 (8), A060672 (9), A060673 (10), A060674 (11), A060675 (12).
%K A060667 nonn
%O A060667 1,1
%A A060667 _Robert G. Wilson v_, Apr 18 2001