A377899 a(n) = number of composite numbers c_{2*k+1} <= n, where c_m = A002808(m) is the m-th composite number.

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

Offset: 1

Author

N. J. A. Sloane, Nov 14 2024

Keywords

• nonn

Examples

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

Crossrefs

Cf. A000720, A002808, A062298, A099861.

Programs

Python

from sympy import primepi
def A377899(n): return n-primepi(n)>>1 # Chai Wah Wu, Nov 14 2024

Formula

a(n) = floor((n-A000720(n))/2). - Chai Wah Wu, Nov 14 2024