revert m
l m : ℕ
hm_pos : 0 < m
⊢ Squarefree m → (∑ d ∈ m.divisors, if l ∣ d then μ d else 0) = if l = m then μ l else 0
l : ℕ
⊢ ∀ (m : ℕ), 0 < m → Squarefree m → (∑ d ∈ m.divisors, if l ∣ d then μ d else 0) = if l = m then μ l else 0