-- Mathlib/FieldTheory/Fixed.lean
theorem linearIndependent_smul_of_linearIndependent {s : Finset F} :
    (LinearIndepOn (FixedPoints.subfield G F) id (s : Set F)) →
      LinearIndepOn F (MulAction.toFun G F) s

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this✝ : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
this : ∀ i ∈ s, l i ∈ subfield G F
hla : ∑ x ∈ s, l x • x = a
c : F
hcs : c ∈ s
⊢ ⟨l c, ⋯⟩ • c ∈ Submodule.span (↥(subfield G F)) (id '' ↑s)

Tactic:
exact Submodule.smul_mem _ _ <| Submodule.subset_span <| by simpa

Result:
no goals

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s : Finset F
⊢ LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s

Tactic:
have : IsEmpty ((∅ : Finset F) : Set F) := by simp

Result:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s : Finset F
this : IsEmpty ↑↑∅
⊢ LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this✝ : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
this : ∀ i ∈ s, l i ∈ subfield G F
hla : ∑ x ∈ s, l x • x = a
⊢ False

Tactic:
refine hs.2 (hla ▸ Submodule.sum_mem _ fun c hcs => ?_)

Result:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this✝ : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
this : ∀ i ∈ s, l i ∈ subfield G F
hla : ∑ x ∈ s, l x • x = a
c : F
hcs : c ∈ s
⊢ l c • c ∈ Submodule.span (↥(subfield G F)) (id '' ↑s)

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this✝ : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
this : ∀ i ∈ s, l i ∈ subfield G F
hla : ∑ x ∈ s, l x • x = a
c : F
hcs : c ∈ s
⊢ c ∈ id '' ↑s

Tactic:
simpa

Result:
no goals

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
ha : ∃ l ∈ Finsupp.supported F F ↑s, (Finsupp.linearCombination F ⇑(toFun G F)) l = (toFun G F) a
⊢ False

Tactic:
rcases ha with ⟨l, hl, hla⟩

Result:
case intro.intro
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
hla : (Finsupp.linearCombination F ⇑(toFun G F)) l = (toFun G F) a
⊢ False

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this✝ : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
this : ∀ i ∈ s, l i ∈ subfield G F
hla : ∑ x ∈ s, l x • x = a
c : F
hcs : c ∈ s
⊢ l c • c ∈ Submodule.span (↥(subfield G F)) (id '' ↑s)

Tactic:
change (⟨l c, this c hcs⟩ : FixedPoints.subfield G F) • c ∈ _

Result:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this✝ : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
this : ∀ i ∈ s, l i ∈ subfield G F
hla : ∑ x ∈ s, l x • x = a
c : F
hcs : c ∈ s
⊢ ⟨l c, ⋯⟩ • c ∈ Submodule.span (↥(subfield G F)) (id '' ↑s)

Proof state:
case intro.intro.h
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
hla : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g g' : G
⊢ ∑ x ∈ s, g • (fun y => l y • (toFun G F) y) x (g⁻¹ * g') = ∑ x ∈ s, (fun y => l y • (toFun G F) y) x g'

Tactic:
rw [← smul_sum, ← sum_apply _ _ fun y => l y • toFun G F y, ←
  sum_apply _ _ fun y => l y • toFun G F y]

Result:
case intro.intro.h
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
hla : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g g' : G
⊢ g • (∑ c ∈ s, l c • (toFun G F) c) (g⁻¹ * g') = (∑ c ∈ s, l c • (toFun G F) c) g'

Proof state:
case intro.intro.h
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
hla : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g g' : G
⊢ ∑ x ∈ s, g • (l x * (toFun G F) x (g⁻¹ * g')) = ∑ x ∈ s, l x • (toFun G F) x g'

Tactic:
show
  (∑ x ∈ s, g • (fun y => l y • MulAction.toFun G F y) x (g⁻¹ * g')) =
    ∑ x ∈ s, (fun y => l y • MulAction.toFun G F y) x g'

Result:
case intro.intro.h
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s✝ : Finset F
this : IsEmpty ↑↑∅
a : F
s : Finset F
has : a ∉ s
ih : LinearIndepOn (↥(subfield G F)) id ↑s → LinearIndepOn F ⇑(toFun G F) ↑s
hs : LinearIndepOn (↥(subfield G F)) id ↑s ∧ id a ∉ Submodule.span (↥(subfield G F)) (id '' ↑s)
l : F →₀ F
hl : l ∈ Finsupp.supported F F ↑s
hla : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g g' : G
⊢ ∑ x ∈ s, g • (fun y => l y • (toFun G F) y) x (g⁻¹ * g') = ∑ x ∈ s, (fun y => l y • (toFun G F) y) x g'