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 A139208 #7 Dec 18 2018 12:03:16
%S A139208 1,3,7,15,21,31,45,63,93,105,127,189,217,255,315,341,455,511,819,1023,
%T A139208 1365,2047,3591,3855,4095,5461,8191,12483,13107,16383,21845,29127,
%U A139208 32767,53261,55831,60787,65535,87381,131071,178481,182361,209715,258111,262143,349525,430185,479349,524287
%N A139208 Numbers 2*k+1 for which numbers A006694(k) are record values for A006694.
%C A139208 Question. Is it true that all primes in this sequence are Mersenne primes?
%C A139208 178481 is a prime term, but is not a Mersenne prime. - _Michel Marcus_, Dec 18 2018
%o A139208 (PARI) a006694(n)=sumdiv(2*n+1, d, eulerphi(d)/znorder(Mod(2, d))) - 1;
%o A139208 lista(nn) = {my(m = -1, newm); for(n=0, nn, newm = a006694(n); if (newm > m, m = newm; print1(2*n+1, ", ")););} \\ _Michel Marcus_, Dec 18 2018
%Y A139208 Cf. A006694, A000668.
%K A139208 nonn
%O A139208 1,2
%A A139208 _Vladimir Shevelev_, Jun 06 2008
%E A139208 Offset 1, missing term 819 and more terms from _Michel Marcus_, Dec 18 2018