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A122170 Number of primes p <= 2n such that p+n is also a prime. 1

%I

%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.

%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.

%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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Last modified August 9 04:39 EDT 2020. Contains 336319 sequences. (Running on oeis4.)