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 A109550 #8 Aug 03 2014 14:26:15
%S A109550 31,41,61,71,331,661,3331,4441,6661,33331,333331,3333331,33333331,
%T A109550 6666666661,44444444441,555555555551,5555555555551,7777777777771,
%U A109550 333333333333333331,666666666666666661,77777777777777777771
%N A109550 Primes of the form aaaa...aa1 where a is 3, 4, 5, 6 or 7.
%H A109550 Vincenzo Librandi, <a href="/A109550/b109550.txt">Table of n, a(n) for n = 1..52</a>
%F A109550 d=3, 4, 5, 6, 7 a(n) = if prime then Sum[d*10^i, {i, 1, m}] + 1
%t A109550 d[n_] = If[2 + Mod[n, 6] > 0, 2 + 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 A109550 Select[FromDigits/@Flatten[Table[PadLeft[{1},i,#]&/@{3,4, 5,6,7},{i,2,100}],1],PrimeQ[#]&] (* _Vincenzo Librandi_, Dec 12 2011 *)
%Y A109550 Cf. A051200, A004022, A093176, A093177, A089346, A093174, A092571, A089345, A089347.
%K A109550 nonn
%O A109550 1,1
%A A109550 _Roger L. Bagula_, Jun 26 2005