cp's OEIS Frontend

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

%I A178000 #15 Jan 12 2019 02:27:19
%S A178000 2,29,229,2221,22229,922223,9222229,22222223,222222227,7222222229,
%T A178000 22222222223,522222222229,9222222222229,22222222222229,
%U A178000 222222222222227,9222222222222227,72222222222222221,222222222222222221
%N A178000 Largest n-digit prime with the maximum number of digits equal to 2.
%C A178000 Select first for most 2's, then take the largest.
%C A178000 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
%C A178000 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
%H A178000 Robert Israel, <a href="/A178000/b178000.txt">Table of n, a(n) for n = 1..999</a>
%p A178000 # This program will return FAIL if a(n) has more than two digits <> 2.
%p A178000 f:= proc(n)
%p A178000    local k1,d2,k2,t;
%p A178000    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;
%p A178000    for d2 from n to 2 by -1 do
%p A178000      for k2 in [9,8,7,6,5,4,3] do
%p A178000        for k1 in [9,7,3,1] do
%p A178000          t:= 2/9*(10^(n)-1)+(k2-2)*10^(d2-1) + k1-2;
%p A178000          if isprime(t) then return t fi;
%p A178000   od od od:
%p A178000   FAIL
%p A178000 end proc:
%p A178000 f(1):= 2:
%p A178000 seq(f(n),n=1..30); # _Robert Israel_, Mar 20 2018
%Y A178000 Cf. A037057, A099656, A177999.
%K A178000 nonn,base
%O A178000 1,1
%A A178000 _Lekraj Beedassy_, May 17 2010