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

Offset: 1

Author

Reinhard Zumkeller, Feb 18 2009

Keywords

• nonn

Comments

a(n) = SUM(A156659(k): 1<=k<=n);

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

Examples

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

Links

• R. Zumkeller, Table of n, a(n) for n = 1..10000
• Wikipedia, Safe prime

Crossrefs

A005385, A156874, A156876, A156877, A156878, A000720.