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A377899
a(n) = number of composite numbers c_{2*k+1} <= n, where c_m = A002808(m) is the m-th composite number.
2
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
1,8
FORMULA
a(n) = floor((n-A000720(n))/2). - Chai Wah Wu, Nov 14 2024
EXAMPLE
c_1 = 4 and c_3 = 8 are <= 9, so a(9) = 2.
PROG
(Python)
from sympy import primepi
def A377899(n): return n-primepi(n)>>1 # Chai Wah Wu, Nov 14 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Nov 14 2024
STATUS
approved