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 A271314 #24 Feb 25 2024 02:16:31
%S A271314 3,5,11,7,43,17,19,31,683,13,2731,127,331,257,43691,73,174763,41,5419,
%T A271314 683,2796203,241,4051,8191,87211,127,3033169,331,715827883,65537,
%U A271314 20857,131071,86171,109,25781083,524287,22366891,61681,8831418697,5419,2932031007403,2113,18837001
%N A271314 Largest prime factor of the n-th Jacobsthal number, A001045(n).
%C A271314 a(22) = 683 is the first repeated term in this sequence. Note that a(n+2) = A129738(n), for n < 20.
%H A271314 Amiram Eldar, <a href="/A271314/b271314.txt">Table of n, a(n) for n = 3..1122</a> (terms 3..988 from Charles R Greathouse IV)
%e A271314 a(6) = 7 because A001045(6) = 21 = 3*7.
%t A271314 FactorInteger[#][[-1, 1]] & /@ Take[#, -(Length@ # - 3)] &@ CoefficientList[Series[x/(1 - x - 2 x^2), {x, 0, 45}], x] (* _Michael De Vlieger_, Apr 04 2016, after _Robert G. Wilson v_ at A001045 *)
%o A271314 (PARI) a001045(n) = (2^n - (-1)^n) / 3;
%o A271314 a(n) = vecmax(factor(a001045(n))[,1]);
%Y A271314 Essentially a combination of A005420 and A002587.
%Y A271314 Cf. A001045, A006530, A060385, A129738, A264137.
%K A271314 nonn
%O A271314 3,1
%A A271314 _Altug Alkan_, Apr 03 2016