This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A345220 #17 Jul 05 2025 10:56:40
%S A345220 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 A345220 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 A345220 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 A345220 Number of divisors d of n with an even number of primes not exceeding d.
%C A345220 Inverse Möbius transform of (pi(n)+1) mod 2 = A131377(n). - _Wesley Ivan Hurt_, Jul 04 2025
%F A345220 a(n) = Sum_{d|n} ((pi(d)+1) mod 2).
%F A345220 a(n) = A000005(n) - A345219(n). - _Wesley Ivan Hurt_, Jul 05 2025
%e A345220 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 A345220 Table[Sum[Mod[PrimePi[d] + 1, 2], {d, Divisors[n]}], {n, 100}]
%o A345220 (PARI) a(n) = sumdiv(n, d, !(primepi(d) % 2)); \\ _Michel Marcus_, Jun 11 2021
%Y A345220 Cf. A000005 (tau), A000720 (pi), A131377, A345219.
%K A345220 nonn
%O A345220 1,3
%A A345220 _Wesley Ivan Hurt_, Jun 11 2021