A156875 - OEIS

A156875

Number of safe primes <= n.

0, 0, 0, 0, 1, 1, 2, 2, 2, 2, 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, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7

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OFFSET

1,7

LINKS

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

Wikipedia, Safe and Sophie Germain primes.

FORMULA

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

a(n) = A156874(floor((n-1)/2)).

EXAMPLE

a(120) = #{5, 7, 11, 23, 47, 59, 83, 107} = 8.

MATHEMATICA