login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Number of primes <= n that are neither safe primes and nor Sophie Germain primes.
5

%I #9 Nov 06 2022 07:48:13

%S 0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,2,2,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,

%T 4,4,5,5,5,5,5,5,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,

%U 8,8,9,9,10,10,10,10,10,10,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11

%N Number of primes <= n that are neither safe primes and nor Sophie Germain primes.

%H Reinhard Zumkeller, <a href="/A156878/b156878.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A000720(n) - A156876(n).

%e a(120) = #{13,17,19,31,37,43,61,67,71,73,79,97,10,103,109} = 15.

%Y Cf. A059500, A005384, A005385.

%Y Cf. A000720, A156876.

%K nonn

%O 1,17

%A _Reinhard Zumkeller_, Feb 18 2009