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%I #10 Mar 22 2023 13:33:37
%S 1,1,1,2,1,3,0,3,1,4,1,5,0,4,1,4,1,6,0,4,1,4,0,10,0,6,1,5,1,12,0,5,0,
%T 6,1,13,0,7,1,9,1,13,0,7,1,6,0,13,0,9,1,7,0,14,0,12,1,7,1,19,0,7,0,10,
%U 1,20,0,11,1,13,1,15,0,8,0,10,1,18,0,12,1,8,0,23,0,10,1,10,0,26,0,13,0,13
%N Number of primes p <= 2n such that p+n is also a prime.
%C a(n)=0 for n in A007921.
%H Robert Israel, <a href="/A122170/b122170.txt">Table of n, a(n) for n = 1..10000</a>
%e a(12)=5 because only the 5 primes p=5,7,11,17,19 below 24 form other primes p+12 = 17,19,23,29,31.
%p P:= select(isprime, [2,seq(i,i=3..3000,2)]);
%p f:= proc(n) local m,R;
%p m:= ListTools:-BinaryPlace(P,3*n);
%p R:= convert(P[1..m],set);
%p nops((R -~ n) intersect R)
%p end proc:
%p f(1):= 1:
%p map(f, [$1..1000]); # _Robert Israel_, Mar 22 2023
%t Table[Length[Select[Select[Range[2*n], PrimeQ], PrimeQ[ #+n]&]], {n, 100}] (* _Ryan Propper_, Nov 12 2006 *)
%K nonn
%O 1,4
%A _Lekraj Beedassy_, Aug 23 2006
%E More terms from _Ryan Propper_, Nov 12 2006
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