apply sum_congr rfl
s : Sieve
d : ℕ
hd : d ∣ P
⊢ (∑ l ∈ P.divisors, if l ∣ d then g (d / l) else 0) = ∑ i ∈ P.divisors, g d * if i ∣ d then 1 / g i else 0
s : Sieve
d : ℕ
hd : d ∣ P
⊢ ∀ x ∈ P.divisors, (if x ∣ d then g (d / x) else 0) = g d * if x ∣ d then 1 / g x else 0