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 A377899 #11 Nov 15 2024 09:03:43
%S A377899 0,0,0,1,1,1,1,2,2,3,3,3,3,4,4,5,5,5,5,6,6,7,7,7,8,8,9,9,9,10,10,10,
%T A377899 11,11,12,12,12,13,13,14,14,14,14,15,15,16,16,16,17,17,18,18,18,19,19,
%U A377899 20,20,21,21,21,21,22,22,23,23,24,24,24,25,25,25,26,26,26,27,27,28,28,28,29,29,30,30,30,31,31,32,32,32,33,33,34,34,35,35,36,36,36,37,37,37
%N A377899 a(n) = number of composite numbers c_{2*k+1} <= n, where c_m = A002808(m) is the m-th composite number.
%F A377899 a(n) = floor((n-A000720(n))/2). - _Chai Wah Wu_, Nov 14 2024
%e A377899 c_1 = 4 and c_3 = 8 are <= 9, so a(9) = 2.
%o A377899 (Python)
%o A377899 from sympy import primepi
%o A377899 def A377899(n): return n-primepi(n)>>1 # _Chai Wah Wu_, Nov 14 2024
%Y A377899 Cf. A000720, A002808, A062298, A099861.
%K A377899 nonn
%O A377899 1,8
%A A377899 _N. J. A. Sloane_, Nov 14 2024