rw [if_neg hl, sum_eq_zero]
case neg
l m : ℕ
hm_pos : m > 0
hm : m ∈ {n | Squarefree n}
hl : ¬l ∣ m
⊢ (∑ i ∈ m.divisors, μ (m / i) • if l = i then μ l else 0) = if l ∣ m then μ m else 0
case neg
l m : ℕ
hm_pos : m > 0
hm : m ∈ {n | Squarefree n}
hl : ¬l ∣ m
⊢ ∀ x ∈ m.divisors, (μ (m / x) • if l = x then μ l else 0) = 0