have hm_pos : 0 < m := Nat.pos_of_ne_zero $ Squarefree.ne_zero hm
l m : ℕ
hm : Squarefree m
⊢ (∑ d ∈ m.divisors, if l ∣ d then μ d else 0) = if l = m then μ l else 0
l m : ℕ
hm : Squarefree m
hm_pos : 0 < m
⊢ (∑ d ∈ m.divisors, if l ∣ d then μ d else 0) = if l = m then μ l else 0