rw [←sum_filter, Nat.divisors_filter_dvd_of_dvd s.prodPrimes_ne_zero hdP]
case a
s : Sieve
d : ℕ
hdP : d ∣ P
⊢ (∑ l ∈ P.divisors, if l ∣ d then 1 / g l else 0) = (ν d)⁻¹
case a
s : Sieve
d : ℕ
hdP : d ∣ P
⊢ ∑ a ∈ d.divisors, 1 / g a = (ν d)⁻¹