intro a ha
n : ℕ
hn : Squarefree n
⊢ ∀ (a : Fin 3 → ℕ) (ha : a ∈ finMulAntidiagonal 3 n),
    Nat.f a ha ∈
      filter
        (fun x =>
          match x with
          | (x, y) => x.lcm y = n)
        (n.divisors ×ˢ n.divisors)
n : ℕ
hn : Squarefree n
a : Fin 3 → ℕ
ha : a ∈ finMulAntidiagonal 3 n
⊢ Nat.f a ha ∈
    filter
      (fun x =>
        match x with
        | (x, y) => x.lcm y = n)
      (n.divisors ×ˢ n.divisors)