A268705 Smallest n-digit prime having at least n-1 digits equal to 7.

2, 17, 277, 1777, 47777, 727777, 7477777, 77767777, 577777777, 1777777777, 67777777777, 377777777777, 7177777777777, 17777777777777, 577777777777777, 2777777777777777, 77777767777777777, 377777777777777777, 2777777777777777777, 71777777777777777777

Offset: 1

Author

Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, Feb 11 2016

Keywords

• nonn
• base

Links

• Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1125

Crossrefs

Cf. A037067, A241100, A268702 - A268706, A268707, A241206, A266146.

Programs

Mathematica

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]
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 *)