

A156874


Number of Sophie Germain primes <= n.


12



0, 1, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10
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OFFSET

1,3


COMMENTS

a(n) = Sum_{k=1..n} A156660(k).
a(n) = A156875(2*n+1).
HardyLittlewood conjecture: a(n) ~ 2*C2*n/(log(n))^2, where C2=0.6601618158... is the twin prime constant (see A005597).
The truth of the above conjecture would imply that there exists an infinity of Sophie Germain primes (which is also conjectured).
a(n) ~ 2*C2*n/(log(n))^2 is also conjectured by HardyLittlewood for the number of twin primes <= n.


LINKS

R. Zumkeller, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Sophie Germain prime
Wikipedia, Sophie Germain prime


FORMULA

a(10^n)= A092816(n).  Enrique Pérez Herrero, Apr 26 2012


EXAMPLE

a(120) = #{2, 3, 5, 11, 23, 29, 41, 53, 83, 89, 113} = 11.


MATHEMATICA

Accumulate[Table[Boole[PrimeQ[n]&&PrimeQ[2n+1]], {n, 1, 200}]] (* Enrique Pérez Herrero, Apr 26 2012 *)


CROSSREFS

A156875, A156876, A156877, A156878, A000720.
Cf. A005384 Sophie Germain primes p: 2p+1 is also prime.
Cf. A092816.
Sequence in context: A071578 A157791 A236857 * A294234 A078767 A331137
Adjacent sequences: A156871 A156872 A156873 * A156875 A156876 A156877


KEYWORD

nonn


AUTHOR

Reinhard Zumkeller, Feb 18 2009


EXTENSIONS

Edited and commented by Daniel Forgues, Jul 31 2009


STATUS

approved



