A156877 - OEIS

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A156877

Number of primes <= n that are safe primes and also Sophie Germain primes.

5

0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4

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OFFSET

1,11

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A156874(n) + A156875(n) - A156876(n).

a(n) = Sum_{k=1..n} A156659(k)*A156660(k).

EXAMPLE

a(120) = #{5, 11, 23, 83} = 4.

CROSSREFS

Cf. A059455, A005384, A005385, A000720.

Cf. A156874, A156875, A156876, A156659, A156660.

Sequence in context: A074594 A211667 A001069 * A110591 A105209 A179076

Adjacent sequences: A156874 A156875 A156876 * A156878 A156879 A156880

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Feb 18 2009

STATUS

approved