A377899 - OEIS

%I #11 Nov 15 2024 09:03:43

%S 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 11,11,12,12,12,13,13,14,14,14,14,15,15,16,16,16,17,17,18,18,18,19,19,

%U 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 a(n) = number of composite numbers c_{2*k+1} <= n, where c_m = A002808(m) is the m-th composite number.

%F a(n) = floor((n-A000720(n))/2). - _Chai Wah Wu_, Nov 14 2024

%e c_1 = 4 and c_3 = 8 are <= 9, so a(9) = 2.

%o (Python)

%o from sympy import primepi

%o def A377899(n): return n-primepi(n)>>1 # _Chai Wah Wu_, Nov 14 2024

%Y Cf. A000720, A002808, A062298, A099861.

%K nonn

%O 1,8

%A _N. J. A. Sloane_, Nov 14 2024