Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #14 Sep 20 2024 23:54:56
%S 37,73,33373777,33773737,37373773,37377337,73337377,73337773,73373737,
%T 73773373,77337373,77733373,3337737737,3337777337,3377733773,
%U 3733377377,3733733777,3737733773,3773333777,3773337377,3773733737,3777333773,3777337337,3777337373
%N Primes consisting only of digits 3 and 7 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="/A087531/b087531.txt">Table of n, a(n) for n = 1..9185</a>
%t Select[Flatten[Table[FromDigits/@Permutations[Join[Table[3, {i}], Table[7, {i}]]], {i, 7}]], PrimeQ] (* from _Harvey P. Dale_ *)
%o (PARI) \\ Needs B() from A087510.
%o concat(vector(6,k,B(k,3,7,isprime))) \\ _Andrew Howroyd_, Sep 20 2024
%Y Cf. A087510, A087511, A087530.
%K base,nonn
%O 1,1
%A _Amarnath Murthy_ and _Paul D. Hanna_, Sep 12 2003
%E Edited by _Charles R Greathouse IV_, Oct 28 2009
%E Offset changed and a(23) onwards from _Andrew Howroyd_, Sep 20 2024