-- Mathlib/RingTheory/Adjoin/Basic.lean
theorem pow_smul_mem_of_smul_subset_of_mem_adjoin [CommSemiring B] [Algebra R B] [Algebra A B]
    [IsScalarTower R A B] (r : A) (s : Set B) (B' : Subalgebra R B) (hs : r • s ⊆ B') {x : B}
    (hx : x ∈ adjoin R s) (hr : algebraMap A B r ∈ B') : ∃ n₀ : ℕ, ∀ n ≥ n₀, r ^ n • x ∈ B'

Proof state:
case h
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
n : ℕ
hn : n ≥ l.support.sup n₁
a : ↑↑(Submonoid.closure s)
ha : a ∈ l.support
this : n ≥ n₁ a
⊢ (algebraMap A B) r ^ (n - n₁ a) * l a • r ^ n₁ a • ↑a ∈ Subalgebra.toSubmodule B'

Tactic:
apply Subalgebra.mul_mem _ (Subalgebra.pow_mem _ hr _) _

Result:
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
n : ℕ
hn : n ≥ l.support.sup n₁
a : ↑↑(Submonoid.closure s)
ha : a ∈ l.support
this : n ≥ n₁ a
⊢ l a • r ^ n₁ a • ↑a ∈ B'

Proof state:
case intro
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
⊢ ∃ n₀, ∀ n ≥ n₀, (r ^ n • l.sum fun i c => c • ↑i) ∈ B'

Tactic:
choose n₁ n₂ using fun x : Submonoid.closure s => Submonoid.pow_smul_mem_closure_smul r s x.prop

Result:
case intro
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
⊢ ∃ n₀, ∀ n ≥ n₀, (r ^ n • l.sum fun i c => c • ↑i) ∈ B'

Proof state:
case h
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
n : ℕ
hn : n ≥ l.support.sup n₁
a : ↑↑(Submonoid.closure s)
ha : a ∈ l.support
⊢ (fun a b => r ^ n • b • ↑a) a (l a) ∈ Subalgebra.toSubmodule B'

Tactic:
have : n ≥ n₁ a := le_trans (Finset.le_sup ha) hn

Result:
case h
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
n : ℕ
hn : n ≥ l.support.sup n₁
a : ↑↑(Submonoid.closure s)
ha : a ∈ l.support
this : n ≥ n₁ a
⊢ (fun a b => r ^ n • b • ↑a) a (l a) ∈ Subalgebra.toSubmodule B'

Proof state:
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
x : B
hr : (algebraMap A B) r ∈ B'
hx : ∃ l, (Finsupp.linearCombination R Subtype.val) l = x
⊢ ∃ n₀, ∀ n ≥ n₀, r ^ n • x ∈ B'

Tactic:
rcases hx with ⟨l, rfl : (l.sum fun (i : Submonoid.closure s) (c : R) => c • (i : B)) = x⟩

Result:
case intro
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
⊢ ∃ n₀, ∀ n ≥ n₀, (r ^ n • l.sum fun i c => c • ↑i) ∈ B'

Proof state:
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
n : ℕ
hn : n ≥ l.support.sup n₁
a : ↑↑(Submonoid.closure s)
ha : a ∈ l.support
this : n ≥ n₁ a
⊢ r ^ n₁ a • ↑a ∈ B'

Tactic:
change _ ∈ B'.toSubmonoid

Result:
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
n : ℕ
hn : n ≥ l.support.sup n₁
a : ↑↑(Submonoid.closure s)
ha : a ∈ l.support
this : n ≥ n₁ a
⊢ r ^ n₁ a • ↑a ∈ B'.toSubmonoid

Proof state:
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
x : B
hr : (algebraMap A B) r ∈ B'
hx : x ∈ Subalgebra.toSubmodule (adjoin R s)
⊢ ∃ n₀, ∀ n ≥ n₀, r ^ n • x ∈ B'

Tactic:
rw [adjoin_eq_span, Finsupp.mem_span_iff_linearCombination] at hx

Result:
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
x : B
hr : (algebraMap A B) r ∈ B'
hx : ∃ l, (Finsupp.linearCombination R Subtype.val) l = x
⊢ ∃ n₀, ∀ n ≥ n₀, r ^ n • x ∈ B'

Proof state:
case intro
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
⊢ ∃ n₀, ∀ n ≥ n₀, (r ^ n • l.sum fun i c => c • ↑i) ∈ B'

Tactic:
use l.support.sup n₁

Result:
case h
R : Type uR
A : Type uA
B : Type uB
inst✝⁶ : CommSemiring R
inst✝⁵ : CommSemiring A
inst✝⁴ : Algebra R A
inst✝³ : CommSemiring B
inst✝² : Algebra R B
inst✝¹ : Algebra A B
inst✝ : IsScalarTower R A B
r : A
s : Set B
B' : Subalgebra R B
hs : r • s ⊆ ↑B'
hr : (algebraMap A B) r ∈ B'
l : ↑↑(Submonoid.closure s) →₀ R
n₁ : ↥(Submonoid.closure s) → ℕ
n₂ : ∀ (x : ↥(Submonoid.closure s)), r ^ n₁ x • ↑x ∈ Submonoid.closure (r • s)
⊢ ∀ n ≥ l.support.sup n₁, (r ^ n • l.sum fun i c => c • ↑i) ∈ B'