A177999 -id:A177999 - cp's OEIS frontend

A268702 Largest n digit prime having at least n-1 digits equal to 1.

7, 71, 911, 8111, 16111, 911111, 1171111, 71111111, 131111111, 1711111111, 31111111111, 311111111111, 5111111111111, 41111111111111, 111151111111111, 5111111111111111, 11111611111111111, 191111111111111111, 2111111111111111111, 11111111611111111111

Offset: 1

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

			a(3) = 911 since 111, 211, 311, ..., 811 are all composites but 911 is prime. Also of the ten primes of 3 digits which contain at least 2 ones, {101, 113, 131, 151, 181, 191, 211, 311, 811, 911}, 911 is the greatest.

Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1000

Cf. A177999, A241100, A268703 - A268707, A241206.

f[n_] := Block[{k = 0, p = {}, r = (10^n - 1)/9, s = Range@ 10 - 2}, While[k < n, AppendTo[p, Select[r + 10^k*s, PrimeQ]]; k++]; p = Max@ Flatten@ p]; Array[f, 20]

a(n) = {p = precprime(10^n-1); while (#select(x->x==1, digits(p)) != n-1, p = precprime(p-1)); p;} \\ Michel Marcus, Feb 21 2016

A178000 Largest n-digit prime with the maximum number of digits equal to 2.

Original entry on oeis.org

2, 29, 229, 2221, 22229, 922223, 9222229, 22222223, 222222227, 7222222229, 22222222223, 522222222229, 9222222222229, 22222222222229, 222222222222227, 9222222222222227, 72222222222222221, 222222222222222221

Offset: 1

Lekraj Beedassy, May 17 2010

Select first for most 2's, then take the largest.
In more detail: To get a(n), look at the list of all the n-digit primes. Suppose k is the maximum number of 2's of any number on the list. Throw out any prime on the list that does not contain k 2's. Then a(n) = maximal number that is left on the list. - N. J. A. Sloane, Mar 20 2018
For n <= 1000, a(n) has at most two non-2's.  What is the first n for which it has more than two? - Robert Israel, Mar 20 2018

Robert Israel, Table of n, a(n) for n = 1..999

Cf. A037057, A099656, A177999.

# This program will return FAIL if a(n) has more than two digits <> 2.
f:= proc(n)
   local k1,d2,k2,t;
   for k1 in [9,7,3,1] do if isprime(2/9*(10^n-1)+k1-2) then return 2/9*(10^n-1)+k1-2 fi od;
   for d2 from n to 2 by -1 do
     for k2 in [9,8,7,6,5,4,3] do
       for k1 in [9,7,3,1] do
         t:= 2/9*(10^(n)-1)+(k2-2)*10^(d2-1) + k1-2;
         if isprime(t) then return t fi;
  od od od:
  FAIL
end proc:
f(1):= 2:
seq(f(n),n=1..30); # Robert Israel, Mar 20 2018

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A268702 Largest n digit prime having at least n-1 digits equal to 1.

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