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