dsimp only at hb
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
n : ℕ
hn : n ≠ 0
b : ℕ × ℕ
hb : b ∈ filter (fun x => x.1.lcm x.2 = n) (n.divisors ×ˢ n.divisors)
⊢ ∃ a, ∃ (ha : a ∈ finMulAntidiagonal 3 n), Nat.f a ha = b