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