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 A345219 #19 Jul 05 2025 10:59:42
%S A345219 0,1,0,1,1,2,0,1,0,2,1,3,0,1,1,1,1,3,0,2,0,2,1,4,2,2,1,2,0,3,1,2,2,3,
%T A345219 2,5,0,1,0,2,1,3,0,2,1,2,1,5,1,4,2,3,0,4,2,2,0,1,1,5,0,2,0,2,1,4,1,4,
%U A345219 2,4,0,6,1,2,3,2,2,4,0,2,1,2,1,6,3,2,1,3,0,4,0,2,1
%N A345219 Number of divisors d of n with an odd number of primes not exceeding d.
%C A345219 Inverse Möbius transform of pi(n) mod 2 = A071986(n). - _Wesley Ivan Hurt_, Jul 04 2025
%F A345219 a(n) = Sum_{d|n} (pi(d) mod 2).
%F A345219 a(n) = A000005(n) - A345220(n). - _Wesley Ivan Hurt_, Jul 05 2025
%e A345219 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 odd values of pi(d).
%t A345219 Table[Sum[Mod[PrimePi[d], 2], {d, Divisors[n]}], {n, 100}]
%o A345219 (PARI) a(n) = sumdiv(n, d, primepi(d) % 2); \\ _Michel Marcus_, Jun 11 2021
%Y A345219 Cf. A000005 (tau), A000720 (pi), A071986, A345220.
%K A345219 nonn
%O A345219 1,6
%A A345219 _Wesley Ivan Hurt_, Jun 11 2021