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 A099801 #23 Nov 28 2019 06:11:24
%S A099801 0,2,3,4,4,5,6,6,7,8,8,9,9,9,10,11,11,11,12,12,13,14,14,15,15,15,16,
%T A099801 16,16,17,18,18,18,19,19,20,21,21,21,22,22,23,23,23,24,24,24,24,25,25,
%U A099801 26,27,27,28,29,29,30,30,30,30,30,30,30,31,31,32,32,32,33,34,34,34,34,34
%N A099801 PrimePi(2n+1), the number of primes less than or equal to 2n+1.
%C A099801 If we drop a(0), for all numbers k(n) [k(n) = 4*ceiling(n/2) + (-1)^n] congruent to -1 or +1 (mod 4) starting with k(n) = {3,5,7,9,11,...}, a(k(n)) is the number of primes up to a(k(n)). - _Daniel Forgues_, Mar 01 2009
%C A099801 For n > 0, equals 1 (to account for the even prime 2 which is not congruent to -1 or +1 (mod 4)) + partial sums of A101264 (for n > 0). - _Daniel Forgues_, Mar 01 2009
%H A099801 Daniel Forgues, <a href="/A099801/b099801.txt">Table of n, a(n) for n = 0..49999</a>
%p A099801 with(numtheory):seq(pi(2*n-1),n=1..86); # _Emeric Deutsch_
%t A099801 Table[PrimePi[2*n + 1], {n, 0, 100}] (* _Wesley Ivan Hurt_, Oct 02 2013 *)
%Y A099801 Bisection of A000720.
%K A099801 nonn,easy
%O A099801 0,2
%A A099801 _N. J. A. Sloane_, Nov 19 2004
%E A099801 More terms from _Emeric Deutsch_, Apr 12 2005