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 A060669 #14 Nov 17 2024 07:19:31
%S A060669 12,16,84,88,112,232,348,408,592,736,760,780,832,952,984,1032,1048,
%T A060669 1068,1128,1232,1272,1312,1332,1428,1432,1488,1552,1608,1692,1912,
%U A060669 2052,2200,2272,2292,2436,2484,2552,2576,2608,2632,2700,2728,2832,2848,3048,3088
%N A060669 Numbers k such that phi(x) = k has exactly 6 solutions.
%H A060669 Amiram Eldar, <a href="/A060669/b060669.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from T. D. Noe)
%H A060669 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%e A060669 12 = phi(13) = phi(21) = phi(26) = phi(28) = phi(36) = phi(42).
%t A060669 a = Table[ 0, {4000} ]; Do[ p = EulerPhi[ n ]; If[ p < 4001, a[ [ p ] ]++ ], {n, 1, 15000} ]; Select[ Range[ 4000 ], a[ [ # ] ] == 6 & ]
%t A060669 Take[Select[Tally[EulerPhi[Range[50000]]],#[[2]]==6&][[All,1]]//Sort,50] (* _Harvey P. Dale_, Sep 15 2016 *)
%o A060669 (PARI) is(n)=sum(i=1,n,eulerphi(i)==n)==6 \\ _Charles R Greathouse IV_, Mar 03 2014
%o A060669 (PARI) is(k) = invphiNum(k) == 6 \\ _Amiram Eldar_, Nov 17 2024, using _Max Alekseyev_'s invphi.gp
%Y A060669 Cf. A000010.
%Y A060669 Number of solutions: A007617 (0), A007366 (2), A007367 (3), A060667 (4), A060668 (5), this sequence (6), A060670 (7), A060671 (8), A060672 (9), A060673 (10), A060674 (11), A060675 (12).
%K A060669 nonn
%O A060669 1,1
%A A060669 _Robert G. Wilson v_, Apr 18 2001