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 A168389 #8 May 11 2019 17:00:51 %S A168389 1,2,3,4,4,5,6,6,7,8,8,9,9,10,11,11,12,12,13,14,14,15,15,16,16,17,18, %T A168389 18,19,19,20,21,21,22,22,23,23,24,24,25,25,26,27,27,28,29,29,30,30,31, %U A168389 31,32,32,33,34,34,35,36,36,37,37,38,38,39,39,40,40,41,42,42,43,44,44,45 %N A168389 a(n) = PrimePi(A147819(n)). %p A168389 A147819 := proc(n) if n = 1 then 2; else for a from procname(n-1)+1 do if isprime(a-1) or isprime(a+1) then if a <> 3 then return a; end if; end if; end do ; end if; end proc: A168389 := proc(n) numtheory[pi](A147819(n)) ; end proc: seq(A168389(n),n=1..100) ; # _R. J. Mathar_, Dec 03 2009 %Y A168389 Cf. A000720, A147819. %K A168389 nonn %O A168389 1,2 %A A168389 _Giovanni Teofilatto_, Nov 24 2009 %E A168389 Extended beyond a(26) by _R. J. Mathar_, Dec 03 2009