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 A138888 #11 Oct 20 2024 03:54:01
%S A138888 1,2,4,6,7,8,9,10,11,12,13,14,15,16,18,19,20,21,22,23,24,25,26,27,28,
%T A138888 29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,
%U A138888 52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73
%N A138888 Non-Fermat numbers.
%C A138888 Numbers that are not members of A000215.
%F A138888 For n > 2, a(n) = n+m+2 if n+m>=4^(2^m) and a(n) = n+m+1 otherwise where m = floor(log_2(log_2(n-1))). - _Chai Wah Wu_, Oct 19 2024
%t A138888 Complement[Range[10000],{3,5,17,257}] (* _Harvey P. Dale_, Aug 04 2023 *)
%o A138888 (Python)
%o A138888 def A138888(n): return n+(m:=((n-1).bit_length()-1).bit_length() if n>2 else 0)+(n+m>=(1<<(1<<m))+1) # _Chai Wah Wu_, Oct 19 2024
%Y A138888 Cf. A000215, A001690, A018252.
%K A138888 easy,nonn
%O A138888 1,2
%A A138888 _Omar E. Pol_, Apr 03 2008