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A122170
Number of primes p <= 2n such that p+n is also a prime.
2
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
OFFSET
1,4
COMMENTS
a(n)=0 for n in A007921.
LINKS
EXAMPLE
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.
MAPLE
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:
map(f, [$1..1000]); # Robert Israel, Mar 22 2023
MATHEMATICA
Table[Length[Select[Select[Range[2*n], PrimeQ], PrimeQ[ #+n]&]], {n, 100}] (* Ryan Propper, Nov 12 2006 *)
CROSSREFS
Sequence in context: A146094 A098035 A079055 * A066029 A141198 A239621
KEYWORD
nonn
AUTHOR
Lekraj Beedassy, Aug 23 2006
EXTENSIONS
More terms from Ryan Propper, Nov 12 2006
STATUS
approved