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Primes consisting only of digits 5 and 9 occurring with equal frequency.
4

%I #14 Sep 20 2024 23:54:36

%S 59,55599959,99555959,5555995999,5559999559,5595959959,5595999559,

%T 5599595959,5599959559,5955559999,5955599959,5959559959,5959599559,

%U 5995559599,5995559959,5995995559,5999555959,9555599959,9555959599,9555995959,9555999559,9559595599

%N Primes consisting only of digits 5 and 9 occurring with equal frequency.

%C 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).

%H Andrew Howroyd, <a href="/A087535/b087535.txt">Table of n, a(n) for n = 1..5353</a>

%t Sort[Flatten[Table[Select[FromDigits/@Permutations[Flatten[Table[{5,9},{n}]]],PrimeQ],{n,5}]]] (* _Harvey P. Dale_, May 31 2013 *)

%o (PARI) \\ Needs B() from A087510.

%o concat(vector(6,k,B(k,5,9,isprime))) \\ _Andrew Howroyd_, Sep 20 2024

%Y Cf. A087510, A087511, A087534.

%K base,nonn

%O 1,1

%A _Paul D. Hanna_ and _Amarnath Murthy_, Sep 12 2003

%E Offset changed and a(20) onwards from _Andrew Howroyd_, Sep 20 2024