This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A089881 #8 Jun 24 2019 04:38:54
%S A089881 0,0,0,0,2,2,3,3,3,3,3,3,5,5,5,5,5,5,7,7,7,7,7,7,7,7,7,7,7,7,9,9,9,9,
%T A089881 9,9,9,9,9,9,9,9,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,11,
%U A089881 11,13,13,13,13,13,13,13,13,13,13,13,13,15,15,15,15,15,15,15,15,15,15
%N A089881 Number of primes <= n that are members of twin prime pairs.
%C A089881 a(n) = if n<3 then 0 else (if n<7 then A071538(n-2)*2 else A071538(n-2)*2-1).
%e A089881 a(30) = 7 = #{3,5,7,11,13,17,19}, as (3,5), (5,7), (11,13) and (17,19) are all twin prime pairs with members not greater than 30,
%e A089881 a(31) = 9 = #{3,5,7,11,13,17,19,29,31} with two more members for the next twin prime pair (29,31).
%Y A089881 Cf. A001359, A006512, A089882.
%K A089881 nonn
%O A089881 1,5
%A A089881 _Reinhard Zumkeller_, Nov 13 2003