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 A232720 #27 Feb 02 2024 00:03:53 %S A232720 3,5,17,257,65537,83623937,4294967297,6992962672132097 %N A232720 Numbers such that 2*phi(n-2) = n-1. %C A232720 Numbers such that A074057(n) = 0. %C A232720 Note that the first 5 terms are Fermat primes (A019434), while a(6) is prime but is not a Fermat number, and a(7) is (F5 from A000215) but is not prime. Note also that F6, F7, F8 and F9 do not belong to this sequence. %F A232720 a(n) = A050474(n) + 2. - _Jaroslav Krizek_, Feb 25 2015 %o A232720 (PARI) is_A232720(n) = 2*eulerphi(n-2) == n-1 %Y A232720 Cf. A000010, A074057. %K A232720 nonn,more %O A232720 1,1 %A A232720 _Michel Marcus_, Nov 28 2013 %E A232720 a(8) from _Jaroslav Krizek_, Feb 25 2015