apply symm
case a
s : Sieve
l : ℕ
a✝ : l > 0
hlP : l ∈ fun x => ∃ c, P = x * c
⊢ ∑ i ∈ l.divisors, ↑(μ (l / i)) * (ν i)⁻¹ = 1 / g l
case a.a
s : Sieve
l : ℕ
a✝ : l > 0
hlP : l ∈ fun x => ∃ c, P = x * c
⊢ 1 / g l = ∑ i ∈ l.divisors, ↑(μ (l / i)) * (ν i)⁻¹