A156877 - OEIS

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A156877

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

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

(list; graph; refs; listen; history; text; internal format)

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.