A268706 Largest n-digit prime having at least n-1 digits equal to 7.

7, 97, 977, 7877, 97777, 787777, 7877777, 77777747, 787777777, 8777777777, 79777777777, 777777779777, 7877777777777, 77777779777777, 778777777777777, 8777777777777777, 77797777777777777, 797777777777777777, 7777877777777777777, 97777777777777777777

Offset

1,1

Links

Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1268

Maple

f:= proc(n)
  local r, i, j, p;
  r:= 7*(10^n-1)/9;
  for p in sort([r, seq(seq(r + i*10^j, i=[$(-7)..(-1), 1, 2]), j=0..n-1)], `>`) do
    if isprime(p) then return p fi
  od;
  error("no prime found")
end proc:
map(f, [$1..100]); # Robert Israel, Feb 24 2016

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 = Max@ Flatten@ p]; Array[f, 20]

Crossrefs

Cf. A241100, A268702, A268703, A268704, A268705, A268707, A241206, A266146.

Sequence in context: A362472 A242377 A178005 * A030540 A071361 A007810

Adjacent sequences: A268703 A268704 A268705 * A268707 A268708 A268709

Keyword

nonn,base

Author

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

Status

approved