have hmul : m / l * l = m := Nat.div_mul_cancel hl
case pos
l m : ℕ
hm_pos : m > 0
hm : m ∈ {n | Squarefree n}
hl : l ∣ m
⊢ (μ (m / l) • if l = l then μ l else 0) = μ m
case pos
l m : ℕ
hm_pos : m > 0
hm : m ∈ {n | Squarefree n}
hl : l ∣ m
hmul : m / l * l = m
⊢ (μ (m / l) • if l = l then μ l else 0) = μ m