A087528 Primes consisting only of digits 2 and 9 occurring with equal frequency.

29, 22929299, 29229929, 29299229, 29992229, 92922299, 99292229, 2229922999, 2229929929, 2229992299, 2292229999, 2292929299, 2292999229, 2299222999, 2299292929, 2299922929, 2922929929, 2922992299, 2929292299, 2929299229, 2992299229, 2992922299, 2992992229, 2999229229

Offset: 1

Author

Paul D. Hanna and Amarnath Murthy, Sep 12 2003

Keywords

• base
• nonn

Comments

There are 18 digit pairs which can produce such primes. (1,0),(1,3),(1,4),(1,6),(1,7),(1,9),(2,3),(2,9),(3,4),(3,5),(3,7),(3,8),(4,7),(4,9),(5,9),(6,7),(7,9),(8,9).

The number of digits in a term is even but not a multiple of 6. - Robert Israel, Oct 30 2019

Links

• Robert Israel, Table of n, a(n) for n = 1..6235

Crossrefs

Cf. A087510, A087511, A087527.

Programs

Maple

F:= proc(d) local C,c;
   if d mod 3 = 0 then return NULL fi;
   C:= map(t -> [0,op(t)], combinat:-choose([$1..d-1],d/2-1));
   C:= map(t -> 2*(10^d-1)/9 + 7*add(10^c,c=t), C);
   op(sort(select(isprime,C)))
end proc:
seq(F(d),d=2..14,2); # Robert Israel, Oct 30 2019

PARI

\\ Needs B() from A087510.
concat(vector(6,k,B(k,2,9,isprime))) \\ Andrew Howroyd, Sep 21 2024

Extensions

Offset corrected by Robert Israel, Oct 30 2019