OFFSET
1,4
EXAMPLE
The subsets S of {1,2,3,4,5,6} such that every number in S is a composite are {}, {4}, {6}, and {4,6}, so a(6) = 4.
MATHEMATICA
(1/2) Table[2^(n - PrimePi[n]), {n, 50}]
PROG
(Python)
from sympy import primepi
def a(n): return 2**(n-primepi(n)-1)
print([a(n) for n in range(1, 42)]) # Michael S. Branicky, Sep 24 2022
(PARI) a(n) = 1 << (n-primepi(n)-1); \\ Kevin Ryde, Sep 24 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Sep 24 2022
STATUS
approved