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A156875 Number of safe primes <= n.

%I A156875 #3 Mar 30 2012 18:51:04
%S A156875 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,
%T A156875 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,
%U A156875 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
%N A156875 Number of safe primes <= n.
%C A156875 a(n) = SUM(A156659(k): 1<=k<=n);
%C A156875 a(n) = A156874(floor((n-1)\2)).
%H A156875 R. Zumkeller, <a href="/A156875/b156875.txt">Table of n, a(n) for n = 1..10000</a>
%H A156875 Wikipedia, <a href="http://en.wikipedia.org/wiki/Safe_prime">Safe prime</a>
%e A156875 a(120) = #{5, 7, 11, 23, 47, 59, 83, 107} = 8.
%Y A156875 A005385, A156874, A156876, A156877, A156878, A000720.
%K A156875 nonn
%O A156875 1,7
%A A156875 _Reinhard Zumkeller_, Feb 18 2009