A178007 Largest n-digit prime with the most digits equal to 9.

7, 97, 997, 9949, 99991, 999979, 9999991, 99999989, 999999929, 9999999929, 99999999599, 999999999989, 9999999999799, 99999999999959, 999999999999989, 9999999999999199, 99999999999999997, 999999999999999989, 9999999999999999919, 99999999999999999989, 999999999999999999899, 9999999999999999999929

Offset

1,1

Comments

First maximum the number of 9's, then choose the largest.

From Robert Israel, Dec 18 2024: (Start)

This is believed to be different from A241206, as there should be infinitely many n for which there is no n-digit prime with n-1 digits equal to 9. No examples are known; the least such n is greater than 3400. (End)

Links

Robert Israel, Table of n, a(n) for n = 1..996

Maple

f:= proc(n) local i, j, a, b, x, y;
     x:= 10^n-1;
     for i from 0 to n-1 do
       for a from 1 to 9 do
         y:= x - a*10^i;
         if isprime(y) then return y fi;
     od od;
     for i from 1 to n-1 do
       for a from 1 to 9 do
         for j from 0 to i-1 do
           for b from 1 to 9 do
             y:= x - a*10^i - b*10^j;
             if isprime(y) then return y fi
    od od od od;
    FAIL
end proc:
map(f, [$1..30]); # Robert Israel, Dec 16 2024

Crossrefs

Cf. A037071, A105975, A003618, A241206, A266148.

Sequence in context: A007810 A146758 A114019 * A241206 A127892 A125590

Adjacent sequences: A178004 A178005 A178006 * A178008 A178009 A178010

Keyword

nonn,base

Author

Lekraj Beedassy, May 17 2010

Extensions

Corrected and more terms by Robert Israel, Dec 16 2024

Status

approved