use a 0
case refine_3
n : ℕ
hn : Squarefree n
a : Fin 3 → ℕ
ha : a ∈ finMulAntidiagonal 3 n
⊢ a 1 * a 2 ∣ n
case h
n : ℕ
hn : Squarefree n
a : Fin 3 → ℕ
ha : a ∈ finMulAntidiagonal 3 n
⊢ n = a 1 * a 2 * a 0