/-- If `x * y = a * ∏ i in s, p i` where `p i` is always prime, then
  `x` and `y` can both be written as a divisor of `a` multiplied by
  a product over a subset of `s`  -/
theorem mul_eq_mul_prime_prod {α : Type*} [DecidableEq α] {x y a : R} {s : Finset α} {p : α → R}
    (hp : ∀ i ∈ s, Prime (p i)) (hx : x * y = a * ∏ i in s, p i) :
    ∃ (t u : Finset α) (b c : R),
      t ∪ u = s ∧ Disjoint t u ∧ a = b * c ∧ (x = b * ∏ i in t, p i) ∧ y = c * ∏ i in u, p i := by
  induction' s using Finset.induction with i s his ih generalizing x y a
  · exact ⟨∅, ∅, x, y, by simp [hx]⟩
  · rw [prod_insert his, ← mul_assoc] at hx
    have hpi : Prime (p i) := hp i (mem_insert_self _ _)
    rcases ih (fun i hi ↦ hp i (mem_insert_of_mem hi)) hx with
      ⟨t, u, b, c, htus, htu, hbc, rfl, rfl⟩
    have hit : i ∉ t := fun hit ↦ his (htus ▸ mem_union_left _ hit)
    have hiu : i ∉ u := fun hiu ↦ his (htus ▸ mem_union_right _ hiu)
    obtain ⟨d, rfl⟩ | ⟨d, rfl⟩ : p i ∣ b ∨ p i ∣ c
    

Proof state:
R : Type u_1
inst✝¹ : CancelCommMonoidWithZero R
α : Type u_2
inst✝ : DecidableEq α
p : α → R
i : α
s : Finset α
his : i ∉ s
ih :
  ∀ {x y a : R},
    (∀ i ∈ s, Prime (p i)) →
      x * y = a * ∏ i in s, p i →
        ∃ t u b c, t ∪ u = s ∧ Disjoint t u ∧ a = b * c ∧ x = b * ∏ i in t, p i ∧ y = c * ∏ i in u, p i
a : R
hp : ∀ i_1 ∈ insert i s, Prime (p i_1)
hpi : Prime (p i)
t u : Finset α
b c : R
htus : t ∪ u = s
htu : Disjoint t u
hbc : a * p i = b * c
hx : (b * ∏ i in t, p i) * (c * ∏ i in u, p i) = a * p i * ∏ x in s, p x
hit : i ∉ t
hiu : i ∉ u
⊢ p i ∣ b ∨ p i ∣ c
case insert.intro.intro.intro.intro.intro.intro.intro.intro.inl.intro
R : Type u_1
inst✝¹ : CancelCommMonoidWithZero R
α : Type u_2
inst✝ : DecidableEq α
p : α → R
i : α
s : Finset α
his : i ∉ s
ih :
  ∀ {x y a : R},
    (∀ i ∈ s, Prime (p i)) →
      x * y = a * ∏ i in s, p i →
        ∃ t u b c, t ∪ u = s ∧ Disjoint t u ∧ a = b * c ∧ x = b * ∏ i in t, p i ∧ y = c * ∏ i in u, p i
a : R
hp : ∀ i_1 ∈ insert i s, Prime (p i_1)
hpi : Prime (p i)
t u : Finset α
c : R
htus : t ∪ u = s
htu : Disjoint t u
hit : i ∉ t
hiu : i ∉ u
d : R
hbc : a * p i = p i * d * c
hx : (p i * d * ∏ i in t, p i) * (c * ∏ i in u, p i) = a * p i * ∏ x in s, p x
⊢ ∃ t_1 u_1 b c_1,
    t_1 ∪ u_1 = insert i s ∧
      Disjoint t_1 u_1 ∧
        a = b * c_1 ∧ p i * d * ∏ i in t, p i = b * ∏ i in t_1, p i ∧ c * ∏ i in u, p i = c_1 * ∏ i in u_1, p i
case insert.intro.intro.intro.intro.intro.intro.intro.intro.inr.intro
R : Type u_1
inst✝¹ : CancelCommMonoidWithZero R
α : Type u_2
inst✝ : DecidableEq α
p : α → R
i : α
s : Finset α
his : i ∉ s
ih :
  ∀ {x y a : R},
    (∀ i ∈ s, Prime (p i)) →
      x * y = a * ∏ i in s, p i →
        ∃ t u b c, t ∪ u = s ∧ Disjoint t u ∧ a = b * c ∧ x = b * ∏ i in t, p i ∧ y = c * ∏ i in u, p i
a : R
hp : ∀ i_1 ∈ insert i s, Prime (p i_1)
hpi : Prime (p i)
t u : Finset α
b : R
htus : t ∪ u = s
htu : Disjoint t u
hit : i ∉ t
hiu : i ∉ u
d : R
hbc : a * p i = b * (p i * d)
hx : (b * ∏ i in t, p i) * (p i * d * ∏ i in u, p i) = a * p i * ∏ x in s, p x
⊢ ∃ t_1 u_1 b_1 c,
    t_1 ∪ u_1 = insert i s ∧
      Disjoint t_1 u_1 ∧
        a = b_1 * c ∧ b * ∏ i in t, p i = b_1 * ∏ i in t_1, p i ∧ p i * d * ∏ i in u, p i = c * ∏ i in u_1, p i

Next tactic:
exact hpi.dvd_or_dvd ⟨a, by rw [← hbc, mul_comm]⟩