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 #15 Sep 08 2022 08:46:17
%S 3,27,267,8820000
%N Numbers n such that phi(n^2-1) = phi(n^2+1).
%C No other terms below 10^8. - _Michel Marcus_, Aug 17 2016
%e 3 is a term because phi(3^2-1) = phi(3^2+1).
%t Select[Range@10000000, EulerPhi@(#^2 - 1) == EulerPhi[#^2 + 1] &] (* _Vincenzo Librandi_, Aug 18 2016 *)
%o (PARI) isok(n) = eulerphi(n^2-1) == eulerphi(n^2+1); \\ _Michel Marcus_, Aug 16 2016
%o (Magma) [n: n in [2..100000] | EulerPhi(n^2-1) eq EulerPhi(n^2+1)]; // _Vincenzo Librandi_, Aug 18 2016
%Y Cf. A000010, A066812.
%K nonn,more
%O 1,1
%A _Altug Alkan_, Aug 16 2016