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 A189241 #11 Feb 03 2020 03:50:31
%S A189241 5,7,41,61,73,547,17,19,1181,67,6481,398581,29,31,21523361,103,73,
%T A189241 2851,41,43,5501,23535794707,97,151,53,19441,430697,523,47763361,6883,
%U A189241 926510094425921,67,956353,374857981681,6481,18427,5301533,79,14401
%N A189241 Smallest prime factor of 3^n+1 having the form 2*k*n+1.
%C A189241 The values of k are in A189240.
%H A189241 Amiram Eldar, <a href="/A189241/b189241.txt">Table of n, a(n) for n = 2..658</a>
%e A189241 a(4) = 41 because 3^4 + 1 = 2 * 41 ; the smallest prime divisor of the form 2*k*n+1 is 41 = 2*5*4+1.
%t A189241 Table[p=First/@FactorInteger[3^n+1]; Select[p, Mod[#1, n] == 1 &, 1][[1]],
%t A189241 {n, 2, 40}]
%Y A189241 Cf. A189240, A074476 (largest prime factor of 3^n + 1).
%K A189241 nonn
%O A189241 2,1
%A A189241 _Michel Lagneau_, Apr 19 2011