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A268706 Largest n-digit prime having at least n-1 digits equal to 7. 3
7, 97, 977, 7877, 97777, 787777, 7877777, 77777747, 787777777, 8777777777, 79777777777, 777777779777, 7877777777777, 77777779777777, 778777777777777, 8777777777777777, 77797777777777777, 797777777777777777, 7777877777777777777, 97777777777777777777 (list; graph; refs; listen; history; text; internal format)
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: A289851 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

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Last modified January 27 17:58 EST 2020. Contains 331296 sequences. (Running on oeis4.)