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 A066812 #27 May 07 2025 03:21:44
%S A066812 5,8,9,11,27,33,71,75,123,147,309,315,387,513,555,573,627,636,729,795,
%T A066812 843,911,915,1016,1083,1227,1323,1331,1347,1467,1515,1609,1755,1995,
%U A066812 2133,2171,2187,2307,2403,2427,2475,2591,2643,2696,2763,2907,3243,3315
%N A066812 Numbers k such that phi(k+1) = phi(k-1).
%C A066812 a(n) is even for n = 2,18,24,44,67 while a(n) is odd for other values of n < 85.
%C A066812 a(n) is even for 63 out of the first 10^4 terms. - _Amiram Eldar_, May 07 2025
%H A066812 Amiram Eldar, <a href="/A066812/b066812.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harry J. Smith)
%F A066812 Conjecture: there is a constant 2 < c < 2.1 such that a(n)=n^c asymptotically.
%F A066812 a(n) = A001494(n) + 1.
%t A066812 Flatten[Position[Partition[EulerPhi[Range[3400]],3,1],_?(#[[1]]== #[[3]]&), 1,Heads->False]]+1 (* _Harvey P. Dale_, Apr 18 2018 *)
%o A066812 (PARI) isok(k) = { eulerphi(k+1) == eulerphi(k-1) } \\ _Harry J. Smith_, Mar 29 2010
%Y A066812 Cf. A000010 (phi), A001494.
%K A066812 nonn
%O A066812 1,1
%A A066812 _Benoit Cloitre_, Jan 20 2002
%E A066812 Corrected by _Ralf Stephan_, Feb 20 2004