|
%I #14 Jan 23 2021 14:45:42
%S 2,17,277,1777,47777,727777,7477777,77767777,577777777,1777777777,
%T 67777777777,377777777777,7177777777777,17777777777777,
%U 577777777777777,2777777777777777,77777767777777777,377777777777777777,2777777777777777777,71777777777777777777
%N Smallest n-digit prime having at least n-1 digits equal to 7.
%H Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, <a href="/A268705/b268705.txt">Table of n, a(n) for n = 1..1125</a>
%t f[n_] := Block[{k = 0, p = {}, r = 7(10^n - 1)/9, s = Range@ 10 - 8}, While[k < n, AppendTo[p, Select[r + 10^k*s, PrimeQ]]; k++]; p = Min@ Flatten@ p]; f[1] = 2; f[2] = 17; Array[f, 20]
%t Table[Min[Select[FromDigits/@Flatten[Permutations/@Table[Join[ {n},PadRight[ {},k,7]],{n,0,9}],1],IntegerLength[#]==k+1&&PrimeQ[#]&]],{k,0,20}] (* _Harvey P. Dale_, Jan 23 2021 *)
%Y Cf. A037067, A241100, A268702 - A268706, A268707, A241206, A266146.
%K nonn,base
%O 1,1
%A _Michel Lagneau_, _Michael De Vlieger_ and _Robert G. Wilson v_, Feb 11 2016
|
