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A122170
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Number of primes p <= 2n such that p+n is also a prime.
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2
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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, 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, 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
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OFFSET
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1,4
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COMMENTS
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LINKS
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EXAMPLE
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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.
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MAPLE
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P:= select(isprime, [2, seq(i, i=3..3000, 2)]);
f:= proc(n) local m, R;
m:= ListTools:-BinaryPlace(P, 3*n);
R:= convert(P[1..m], set);
nops((R -~ n) intersect R)
end proc:
f(1):= 1:
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MATHEMATICA
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Table[Length[Select[Select[Range[2*n], PrimeQ], PrimeQ[ #+n]&]], {n, 100}] (* Ryan Propper, Nov 12 2006 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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