%I A037067
%S A037067 7,277,1777,47777,727777,7477777,77767777,577777777,1777777777,
%T A037067 67777777777,377777777777,7177777777777,17777777777777,577777777777777,
%U A037067 2777777777777777,77777767777777777,377777777777777777
%N A037067 Smallest prime containing exactly n 7's.
%t A037067 f[n_, b_] := Block[{k = 10^(n + 1), p = Permutations[ Join[ Table[b, 
               {i, 1, n}], {x}]], c = Complement[Table[j, {j, 0, 9}], {b}], q = 
               {}}, Do[q = Append[q, Replace[p, x -> c[[i]], 2]], {i, 1, 9}]; r 
               = Min[ Select[ FromDigits /@ Flatten[q, 1], PrimeQ[ # ] & ]]; If[r 
               ? Infinity, r, p = Permutations[ Join[ Table[ b, {i, 1, n}], {x, 
               y}]]; q = {}; Do[q = Append[q, Replace[p, {x -> c[[i]], y -> c[[j]]}, 
               2]], {i, 1, 9}, {j, 1, 9}]; Min[ Select[ FromDigits /@ Flatten[q, 
               1], PrimeQ[ # ] & ]]]]; Table[ f[n, 7], {n, 1, 18}]
%Y A037067 Cf. A065590, A065581, A037066, A034388, A036507-A036536.
%Y A037067 Cf. A037053, A037055, A037057, A037059, A037061, A037063, A037065, A037069, 
               A037071.
%Y A037067 Sequence in context: A069449 A140031 A066413 this_sequence A065581 A062646 
               A009503
%Y A037067 Adjacent sequences: A037064 A037065 A037066 this_sequence A037068 A037069 
               A037070
%K A037067 nonn,base
%O A037067 1,1
%A A037067 Patrick De Geest (pdg(AT)worldofnumbers.com), Jan 04 1999.
%E A037067 More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Mar 
               23 2003