%I #8 Jun 11 2021 18:31:14
%S 1,1,2,2,1,2,2,3,3,2,1,3,2,3,3,4,1,3,2,4,4,2,1,4,1,2,3,4,2,5,1,4,2,1,
%T 2,4,2,3,4,6,1,5,2,4,5,2,1,5,2,2,2,3,2,4,2,6,4,3,1,7,2,2,6,5,3,4,1,2,
%U 2,4,2,6,1,2,3,4,2,4,2,8,4,2,1,6,1,2,3,5,2,8,4,4,3
%N Number of divisors of n with an even number of primes not exceeding them.
%F a(n) = Sum_{d|n} ((pi(d)+1) mod 2).
%e a(24) = 4; The divisors d of 24 are: 1, 2, 3, 4, 6, 8, 12, 24 and the corresponding values of pi(d) are: 0, 1, 2, 2, 3, 4, 5, 9. There are 4 even values of pi(d).
%t Table[Sum[Mod[PrimePi[k] + 1, 2] (1 - Ceiling[n/k] + Floor[n/k]), {k, n}], {n, 100}]
%o (PARI) a(n) = sumdiv(n, d, !(primepi(d) % 2)); \\ _Michel Marcus_, Jun 11 2021
%Y Cf. A000005, A000720, A345219.
%K nonn
%O 1,3
%A _Wesley Ivan Hurt_, Jun 11 2021