-- Mathlib/RingTheory/UniqueFactorizationDomain/Multiplicative.lean
theorem multiplicative_prime_power {f : α → β} (s : Finset α) (i j : α → ℕ)
    (is_prime : ∀ p ∈ s, Prime p) (is_coprime : ∀ᵉ (p ∈ s) (q ∈ s), p ∣ q → p = q)
    (h1 : ∀ {x y}, IsUnit y → f (x * y) = f x * f y)
    (hpr : ∀ {p} (i : ℕ), Prime p → f (p ^ i) = f p ^ i)
    (hcp : ∀ {x y}, IsRelPrime x y → f (x * y) = f x * f y) :
    f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)

Proof state:
case insert
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
i j : α → ℕ
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
this : DecidableEq α := Classical.decEq α
p : α
s : Finset α
hps : p ∉ s
ih :
  (∀ p ∈ s, Prime p) →
    (∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q) → f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)
is_prime : ∀ p_1 ∈ insert p s, Prime p_1
is_coprime : ∀ p_1 ∈ insert p s, ∀ q ∈ insert p s, p_1 ∣ q → p_1 = q
hpr_p : Prime p
hpr_s : ∀ p ∈ s, Prime p
hcp_p : ∀ (i : α → ℕ), IsRelPrime (p ^ i p) (∏ p' ∈ s, p' ^ i p')
⊢ f (∏ p ∈ insert p s, p ^ (i p + j p)) = f (∏ p ∈ insert p s, p ^ i p) * f (∏ p ∈ insert p s, p ^ j p)

Tactic:
have hcp_s : ∀ᵉ (p ∈ s) (q ∈ s), p ∣ q → p = q := fun p hp q hq =>
  is_coprime p (Finset.mem_insert_of_mem hp) q (Finset.mem_insert_of_mem hq)

Result:
case insert
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
i j : α → ℕ
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
this : DecidableEq α := Classical.decEq α
p : α
s : Finset α
hps : p ∉ s
ih :
  (∀ p ∈ s, Prime p) →
    (∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q) → f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)
is_prime : ∀ p_1 ∈ insert p s, Prime p_1
is_coprime : ∀ p_1 ∈ insert p s, ∀ q ∈ insert p s, p_1 ∣ q → p_1 = q
hpr_p : Prime p
hpr_s : ∀ p ∈ s, Prime p
hcp_p : ∀ (i : α → ℕ), IsRelPrime (p ^ i p) (∏ p' ∈ s, p' ^ i p')
hcp_s : ∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q
⊢ f (∏ p ∈ insert p s, p ^ (i p + j p)) = f (∏ p ∈ insert p s, p ^ i p) * f (∏ p ∈ insert p s, p ^ j p)

Proof state:
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
s : Finset α
i j : α → ℕ
is_prime : ∀ p ∈ s, Prime p
is_coprime : ∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
⊢ f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)

Tactic:
letI := Classical.decEq α

Result:
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
s : Finset α
i j : α → ℕ
is_prime : ∀ p ∈ s, Prime p
is_coprime : ∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
this : DecidableEq α := Classical.decEq α
⊢ f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)

Proof state:
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
s : Finset α
i j : α → ℕ
is_prime : ∀ p ∈ s, Prime p
is_coprime : ∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
this : DecidableEq α := Classical.decEq α
⊢ f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)

Tactic:
induction' s using Finset.induction_on with p s hps ih

Result:
case empty
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
i j : α → ℕ
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
this : DecidableEq α := Classical.decEq α
is_prime : ∀ p ∈ ∅, Prime p
is_coprime : ∀ p ∈ ∅, ∀ q ∈ ∅, p ∣ q → p = q
⊢ f (∏ p ∈ ∅, p ^ (i p + j p)) = f (∏ p ∈ ∅, p ^ i p) * f (∏ p ∈ ∅, p ^ j p)

case insert
α : Type u_1
inst✝² : CancelCommMonoidWithZero α
inst✝¹ : UniqueFactorizationMonoid α
β : Type u_3
inst✝ : CancelCommMonoidWithZero β
f : α → β
i j : α → ℕ
h1 : ∀ {x y : α}, IsUnit y → f (x * y) = f x * f y
hpr : ∀ {p : α} (i : ℕ), Prime p → f (p ^ i) = f p ^ i
hcp : ∀ {x y : α}, IsRelPrime x y → f (x * y) = f x * f y
this : DecidableEq α := Classical.decEq α
p : α
s : Finset α
hps : p ∉ s
ih :
  (∀ p ∈ s, Prime p) →
    (∀ p ∈ s, ∀ q ∈ s, p ∣ q → p = q) → f (∏ p ∈ s, p ^ (i p + j p)) = f (∏ p ∈ s, p ^ i p) * f (∏ p ∈ s, p ^ j p)
is_prime : ∀ p_1 ∈ insert p s, Prime p_1
is_coprime : ∀ p_1 ∈ insert p s, ∀ q ∈ insert p s, p_1 ∣ q → p_1 = q
⊢ f (∏ p ∈ insert p s, p ^ (i p + j p)) = f (∏ p ∈ insert p s, p ^ i p) * f (∏ p ∈ insert p s, p ^ j p)