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 A303531 #15 Sep 17 2024 20:54:05
%S A303531 31417,130561,41541241,208969201,224074369,392099401,851934601,
%T A303531 5186365801,33847795741,65096562721,91625968981,104070261901,
%U A303531 111794455501,165814673473,180007280041,184020124201,276032114281,408164260141,620052300421,1240013784241,1244667503017
%N A303531 Numbers k such that both k and (k+1)/2 are Fermat pseudoprimes to base 2 (A001567).
%C A303531 Numbers (k+1)/2 are listed in A298758.
%H A303531 Amiram Eldar, <a href="/A303531/b303531.txt">Table of n, a(n) for n = 1..3031</a> (terms below 2^64)
%F A303531 a(n) = 2*A298758(n) - 1.
%t A303531 Select[Cases[Import["https://oeis.org/A001567/b001567.txt", "Table"], {_, _}][[;; , 2]], !PrimeQ[(#+1)/2] && PowerMod[2, (#-1)/2, (#+1)/2] == 1 &] (* _Amiram Eldar_, Nov 09 2023 *)
%o A303531 (PARI) isF(n) = {Mod(2, n)^n==2 && !isprime(n) && n>1};
%o A303531 isok(n) = (n%2) && isF(n) && isF((n+1)/2); \\ _Michel Marcus_, Apr 26 2018
%Y A303531 Subsequence of each of A001567, A216822, and A217465.
%Y A303531 Cf. A005382, A298758, A303447, A303448.
%K A303531 nonn
%O A303531 1,1
%A A303531 _Max Alekseyev_, Apr 25 2018