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 A140452 #14 Dec 09 2018 10:32:37
%S A140452 11,22,23,25,28,29,33,35,36,37,39,41,43,44,45,46,47,48,50,51,52,53,55,
%T A140452 56,57,58,59,60,63,64,66,67,68,69,70,71,72,73,74,75,76,77,78,79,81,82,
%U A140452 83,84,86,87,88,90,91,92,94,95,96,97,99,100,101,102,103,104,105,106,108,109
%N A140452 2^(a(n))-1 contains an overpseudoprime divisor.
%C A140452 If p is a prime then p is in the sequence iff 2^p-1 is a composite number.
%H A140452 V. Shevelev, <a href="http://arxiv.org/abs/0806.3412">Overpseudoprimes, Mersenne Numbers and Wieferich Primes</a>, arXiv:0806.3412 [math.NT], 2008-2012.
%o A140452 (PARI) f(n) = my(t); sumdiv(2*n+1, d, eulerphi(d)/(t=znorder(Mod(2, d))))*t-t+1; \\ A137576
%o A140452 isopp(n) = (n>1) && !isprime(n) && (n == f((n-1)/2)); \\ A141232
%o A140452 isok(n) = {fordiv(2^n-1, d, if (isopp(d), return (1));); return (0);} \\ _Michel Marcus_, Dec 09 2018
%Y A140452 Cf. A141232, A137576, A005420, A049479.
%K A140452 nonn
%O A140452 1,1
%A A140452 _Vladimir Shevelev_, Jun 26 2008
%E A140452 More terms from _Michel Marcus_, Dec 09 2018