OFFSET
1,2
FORMULA
a(n) = 10^n/2 - A006880(n).
EXAMPLE
a(1) = 1 because 9 is the only odd composite number <= 10^1.
MATHEMATICA
Table[Length@Select[Range[2, 10^n], OddQ@#&&!PrimeQ@#&], {n, 7}]
(* or *)
Table[10^n/2-PrimePi[10^n], {n, 10}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Giorgos Kalogeropoulos, May 26 2021
STATUS
approved