intro b hb
n : ℕ
hn : n ≠ 0
⊢ ∀
    b ∈
      filter
        (fun x =>
          match x with
          | (x, y) => x.lcm y = n)
        (n.divisors ×ˢ n.divisors),
    ∃ a, ∃ (ha : a ∈ finMulAntidiagonal 3 n), Nat.f a ha = b
n : ℕ
hn : n ≠ 0
b : ℕ × ℕ
hb :
  b ∈
    filter
      (fun x =>
        match x with
        | (x, y) => x.lcm y = n)
      (n.divisors ×ˢ n.divisors)
⊢ ∃ a, ∃ (ha : a ∈ finMulAntidiagonal 3 n), Nat.f a ha = b