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 A109549 #8 Aug 03 2014 14:26:15
%S A109549 61,71,661,881,991,6661,99991,9999991,6666666661,7777777777771,
%T A109549 666666666666666661,8888888888888888881,77777777777777777771,
%U A109549 666666666666666666661,6666666666666666666661,77777777777777777777771
%N A109549 Primes of the form aaaa...aa1 where a is 6, 7, 8 or 9.
%C A109549 Easy-to-remember large primes can be formed in this manner.
%H A109549 Vincenzo Librandi, <a href="/A109549/b109549.txt">Table of n, a(n) for n = 1..43</a>
%F A109549 d=6, 7, 8, 9 a(n) = if prime then Sum[d*10^i, {i, 1, m}] + 1
%t A109549 d[n_] = If[5 + Mod[n, 6] > 0, 5 + Mod[n, 6], 1] a = Flatten[Table[Sum[d[k]*10^i, {i, 1, m}] + 1, {m, 1, 50}, {k, 1, 4}]] b = Flatten[Table[If[PrimeQ[a[[i]]] == True, a[[i]], {}], {i, 1, Length[a]}]]
%t A109549 Select[FromDigits/@Flatten[Table[PadLeft[{1},i,#]&/@{6,7,8,9},{i,2,100}],1],PrimeQ[#]&] (* _Vincenzo Librandi_, Dec 12 2011 *)
%Y A109549 Cf. A051200, A004022, A093176, A093177, A089346, A093174, A092571, A089345, A089347.
%K A109549 nonn
%O A109549 1,1
%A A109549 _Roger L. Bagula_, Jun 26 2005