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

1,1

Links

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

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 *)

Crossrefs

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

Sequence in context: A086534 A198287 A338635 * A078367 A090306 A304857

Adjacent sequences: A268702 A268703 A268704 * A268706 A268707 A268708

Keyword

nonn,base

Author

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

Status

approved