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 A303447 #13 Apr 24 2018 15:09:38 %S A303447 9890881,23456248059221,96076792050570581 %N A303447 Numbers m such that both m and 2m+1 are Fermat pseudoprimes base 2 (A001567). %C A303447 No other terms below 2^64. %C A303447 Terms a(2) and a(3) are of the form (4^k-1)/3=A002450(k). Terms A002450(k) belong to this sequence for k in A303009. Correspondingly, a(4) <= A002450(A303009(3)) = (4^41-1)/3 = 1611901092819505566274901. %Y A303447 Numbers 2*a(n)+1 are listed in A303448. %Y A303447 Subsequence of A006970. %Y A303447 Cf. A298758, A300193. %K A303447 bref,hard,more,nonn %O A303447 1,1 %A A303447 _Max Alekseyev_, Apr 24 2018 %E A303447 a(1) from _Amiram Eldar_, Jan 26 2018