cp's OEIS Frontend

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

%I A268706 #12 Mar 12 2016 22:51:16
%S A268706 7,97,977,7877,97777,787777,7877777,77777747,787777777,8777777777,
%T A268706 79777777777,777777779777,7877777777777,77777779777777,
%U A268706 778777777777777,8777777777777777,77797777777777777,797777777777777777,7777877777777777777,97777777777777777777
%N A268706 Largest n-digit prime having at least n-1 digits equal to 7.
%H A268706 Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, <a href="/A268706/b268706.txt">Table of n, a(n) for n = 1..1268</a>
%p A268706 f:= proc(n)
%p A268706   local r,i,j,p;
%p A268706   r:= 7*(10^n-1)/9;
%p A268706   for p in sort([r, seq(seq(r + i*10^j, i=[$(-7)..(-1),1,2]),j=0..n-1)],`>`) do
%p A268706     if isprime(p) then return p fi
%p A268706   od;
%p A268706   error("no prime found")
%p A268706 end proc:
%p A268706 map(f, [$1..100]); # _Robert Israel_, Feb 24 2016
%t A268706 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]
%Y A268706 Cf. A241100, A268702, A268703, A268704, A268705, A268707, A241206, A266146.
%K A268706 nonn,base
%O A268706 1,1
%A A268706 _Michel Lagneau_, _Michael De Vlieger_ and _Robert G. Wilson v_, Feb 11 2016