Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #19 Apr 17 2023 15:35:27
%S 6,12,18,36,72,90,96,108,162,192,432,486,576,702,768,792,924,1152,
%T 1296,1458,2592,2916,3456,3888,4698,5550,6696,7998,8700,10368,10590,
%U 11802,12288,16470,17496,18432,33250,39366,52488,56790,79248,124356
%N Numbers n such that phi(n+1) = 3*phi(n).
%H Donovan Johnson, <a href="/A067143/b067143.txt">Table of n, a(n) for n = 1..1000</a>
%t Position[Partition[EulerPhi[Range[125000]],2,1],_?(#[[2]]==3#[[1]]&), 1,Heads-> False]//Flatten (* _Harvey P. Dale_, Jul 30 2020 *)
%o (PARI) isok(n) = eulerphi(n+1) == 3*eulerphi(n); \\ _Michel Marcus_, Nov 20 2013
%Y Cf. A001274, A050472, A172314.
%K nonn
%O 1,1
%A _Benoit Cloitre_, Feb 19 2002
%E More terms from _Dean Hickerson_, Feb 20 2002