apply (ArithmeticFunction.sum_eq_iff_sum_smul_moebius_eq_on {n | Squarefree n} (fun _ _ => Squarefree.squarefree_of_dvd)).mpr
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
l : ℕ
⊢ ∀ n > 0,
    n ∈ {n | Squarefree n} →
      (∑ x ∈ n.divisorsAntidiagonal, μ x.1 • if l = x.2 then μ l else 0) = if l ∣ n then μ n else 0