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 A200921 #8 Aug 19 2015 09:15:14
%S A200921 1,1,3,3,13,9,15,3,1,13,29,1,5,53,25,9,23,1,69,13,3,3,17,1,5,117,5,13,
%T A200921 45,51,3,11,31,73,49,43,11,83,93,277,171,383,39,11,3,31,55,61,61,13,
%U A200921 73,107,65,137,53,39,467,53,233,277,17,53,109,177,151,97,13
%N A200921 Least k such that (3^n + k)*3^n - 1 is a prime number.
%t A200921 Table[k = 0; While[!PrimeQ[(3^n + k)*3^n - 1], k++]; k, {n, 72}]
%t A200921 lk[n_]:=Module[{c=3^n,k=0},While[!PrimeQ[(c+k)c-1],k++];k]; Array[lk,70] (* _Harvey P. Dale_, Aug 19 2015 *)
%Y A200921 Cf. A191617, A191618, A191619, A191620, A200920, A200922, A200923.
%K A200921 nonn
%O A200921 1,3
%A A200921 _Michel Lagneau_, Nov 24 2011