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 A217468 #27 Dec 29 2017 20:27:28
%S A217468 91625794219,8796093022207,1557609722332488343,18216643597893471403
%N A217468 Composite values of n such that 2^n == 2 (mod n*(n-1)).
%C A217468 No other terms below 2^64.
%C A217468 The next term is <= 25790417485109157029391019 = A019320(258). - _Emmanuel Vantieghem_, Dec 01 2014
%C A217468 Terms A019320(k) belongs to this sequence for k in A297412. - _Max Alekseyev_, Dec 29 2017
%H A217468 Mersenne Forum, <a href="http://mersenneforum.org/showthread.php?t=17198">Prime Conjecture</a>
%e A217468 a(1) = 91625794219 = (2^38 - 2^19 + 1)/3 = A019320(114).
%e A217468 a(2) = 8796093022207 = 2^43 - 1 = A019320(43).
%o A217468 (PARI) is_A217468(n)={Mod(2,(n-1)*n)^n==2 & !isprime(n)} \\ - _M. F. Hasler_, Oct 07 2012
%Y A217468 Cf. A069051, A216822, A219037, A217465.
%Y A217468 Intersection of A001567 and { 2*A014945(k) + 1 }.
%K A217468 nonn,hard,more
%O A217468 1,1
%A A217468 _V. Raman_, Oct 04 2012