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 A078683 #23 Oct 21 2024 00:03:11
%S A078683 3,5,7,17,11,13,29,17,19,41,23,97,53,29,31,257,137,37,1217,41,43,89,
%T A078683 47,97,101,53,109,113,59,61,7937,257,67,137,71,73,149,1217,79,641,83,
%U A078683 337,173,89,181,11777
%N A078683 Least prime of the form n*2^m+1 for m>0, or 0 if there is no such prime.
%C A078683 The next prime has 178 digits. See A078680 for the values of m.
%H A078683 Vincenzo Librandi, <a href="/A078683/b078683.txt">Table of n, a(n) for n = 1..1000</a>
%H A078683 N. J. A. Sloane, <a href="https://www.youtube.com/watch?v=3RAYoaKMckM">A Nasty Surprise in a Sequence and Other OEIS Stories</a>, Experimental Mathematics Seminar, Rutgers University, Oct 10 2024, Youtube video; <a href="https://sites.math.rutgers.edu/~zeilberg/expmath/sloane85BD.pdf">Slides</a> [Mentions this sequence]
%t A078683 Table[m=1; While[! PrimeQ[p = n*2^m + 1], m++]; p, {n, 50}]
%o A078683 (PARI) a(n)=if(n<0, 0, m=1; while(isprime(n*2^m+1)==0, m++); n*2^m+1)
%Y A078683 Cf. A078680.
%K A078683 nonn
%O A078683 1,1
%A A078683 _Benoit Cloitre_, Dec 17 2002
%E A078683 Edited by _T. D. Noe_, Feb 25 2011