cp's OEIS Frontend

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

%I A087528 #16 Sep 21 2024 12:24:11
%S A087528 29,22929299,29229929,29299229,29992229,92922299,99292229,2229922999,
%T A087528 2229929929,2229992299,2292229999,2292929299,2292999229,2299222999,
%U A087528 2299292929,2299922929,2922929929,2922992299,2929292299,2929299229,2992299229,2992922299,2992992229,2999229229
%N A087528 Primes consisting only of digits 2 and 9 occurring with equal frequency.
%C A087528 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).
%C A087528 The number of digits in a term is even but not a multiple of 6. - _Robert Israel_, Oct 30 2019
%H A087528 Robert Israel, <a href="/A087528/b087528.txt">Table of n, a(n) for n = 1..6235</a>
%p A087528 F:= proc(d) local C,c;
%p A087528    if d mod 3 = 0 then return NULL fi;
%p A087528    C:= map(t -> [0,op(t)], combinat:-choose([$1..d-1],d/2-1));
%p A087528    C:= map(t -> 2*(10^d-1)/9 + 7*add(10^c,c=t), C);
%p A087528    op(sort(select(isprime,C)))
%p A087528 end proc:
%p A087528 seq(F(d),d=2..14,2); # _Robert Israel_, Oct 30 2019
%o A087528 (PARI) \\ Needs B() from A087510.
%o A087528 concat(vector(6,k,B(k,2,9,isprime))) \\ _Andrew Howroyd_, Sep 21 2024
%Y A087528 Cf. A087510, A087511, A087527.
%K A087528 base,nonn
%O A087528 1,1
%A A087528 _Paul D. Hanna_ and _Amarnath Murthy_, Sep 12 2003
%E A087528 Offset corrected by _Robert Israel_, Oct 30 2019