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 A033270 #35 Jan 20 2024 17:43:11
%S A033270 0,0,1,1,2,2,3,3,3,3,4,4,5,5,5,5,6,6,7,7,7,7,8,8,8,8,8,8,9,9,10,10,10,
%T A033270 10,10,10,11,11,11,11,12,12,13,13,13,13,14,14,14,14,14,14,15,15,15,15,
%U A033270 15,15,16,16,17,17,17,17
%N A033270 Number of odd primes <= n.
%H A033270 Reinhard Zumkeller, <a href="/A033270/b033270.txt">Table of n, a(n) for n = 1..10000</a>
%F A033270 a(n) = A000720(n) - 1, n > 1. - _Reinhard Zumkeller_, Sep 15 2011
%F A033270 G.f.: (1/(1 - x)) * Sum_{k>=2} x^prime(k). - _Ilya Gutkovskiy_, Sep 14 2019
%t A033270 a[1] = 0; a[n_] := PrimePi[n] - 1; Array[a, 64] (* _Jean-François Alcover_, Nov 28 2017 *)
%o A033270 (Haskell)
%o A033270 a033270 n = a033270_list !! (n-1)
%o A033270 a033270_list = 0 : 0 : scanl1 (+) (drop 2 a010051_list)
%o A033270 -- _Reinhard Zumkeller_, Sep 15 2011
%o A033270 (PARI) a(n)=max(primepi(n)-1,0) \\ _Charles R Greathouse IV_, Jun 25 2013
%o A033270 (Magma) A033270:=func<n|n le 1 select 0 else #PrimesUpTo(n)-1>; // _Jason Kimberley_, May 15 2018
%Y A033270 Cf. A000720, A065091, A304484.
%K A033270 nonn
%O A033270 1,5
%A A033270 _N. J. A. Sloane_