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 A049883 #40 Dec 05 2021 19:13:03
%S A049883 3,5,11,43,683,2731,43691,174763,2796203,715827883,2932031007403,
%T A049883 768614336404564651,201487636602438195784363,
%U A049883 845100400152152934331135470251,56713727820156410577229101238628035243
%N A049883 Primes in the Jacobsthal sequence (A001045).
%C A049883 All terms, except a(2) = 5, are of the form (2^p + 1)/3 - the Wagstaff primes A000979 = {3, 11, 43, 683, 2731, 43691, 174763, ...}.
%C A049883 Indices of prime Jacobsthal numbers are listed in A107036 = {3, 4, 5, 7, 11, 13, 17, 19, 23, 31, 43, 61, ...}.
%C A049883 For n > 1, A107036(n) = A000978(n) (numbers m such that (2^m + 1)/3 is prime). - _Alexander Adamchuk_, Oct 10 2006
%H A049883 Vincenzo Librandi, <a href="/A049883/b049883.txt">Table of n, a(n) for n = 1..23</a>
%t A049883 Select[Table[(2^n + (-1)^(n - 1))/3, {n, 200}], PrimeQ] (* _Vladimir Joseph Stephan Orlovsky_, Mar 29 2011 *)
%Y A049883 Cf. A001045, A000978, A000979, A107036.
%K A049883 nonn
%O A049883 1,1
%A A049883 _Judson Neer_