-- 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:
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 : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g : G
⊢ ∑ x ∈ s, (g • l x - l x) • (toFun G F) x = 0

Tactic:
ext 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 • l x - l x) • (toFun G F) x) g' = 0 g'

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
hla : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
this : ∀ i ∈ s, l i ∈ subfield G F
⊢ False

Tactic:
replace hla := (sum_apply _ _ fun i => l i • toFun G F i).symm.trans (congr_fun hla 1)

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 : ∑ c ∈ s, (l c • (toFun G F) c) 1 = (toFun G F) a 1
⊢ False

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
⊢ g • (∑ c ∈ s, l c • (toFun G F) c) (g⁻¹ * g') = (∑ c ∈ s, l c • (toFun G F) c) g'

Tactic:
rw [hla, toFun_apply, toFun_apply, smul_smul, mul_inv_cancel_left]

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 ↑(insert a s)
⊢ LinearIndepOn F ⇑(toFun G F) ↑(insert a s)

Tactic:
rw [coe_insert] at hs ⊢

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 (insert a ↑s)
⊢ LinearIndepOn F (⇑(toFun G F)) (insert a ↑s)

Proof state:
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 : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
⊢ False

Tactic:
suffices ∀ i ∈ s, l i ∈ FixedPoints.subfield G F by
  replace hla := (sum_apply _ _ fun i => l i • toFun G F i).symm.trans (congr_fun hla 1)
  simp_rw [Pi.smul_apply, toFun_apply, one_smul] at hla
  refine hs.2 (hla ▸ Submodule.sum_mem _ fun c hcs => ?_)
  change (⟨l c, this c hcs⟩ : FixedPoints.subfield G F) • c ∈ _
  exact Submodule.smul_mem _ _ <| Submodule.subset_span <| by simpa

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 : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
⊢ ∀ i ∈ s, l i ∈ subfield G F

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)
⊢ LinearIndepOn F ⇑(toFun G F) ↑s ∧ (toFun G F) a ∉ Submodule.span F (⇑(toFun G F) '' ↑s)

Tactic:
refine ⟨ih hs.1, fun ha => ?_⟩

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)
ha : (toFun G F) a ∈ Submodule.span F (⇑(toFun G F) '' ↑s)
⊢ False

Proof state:
G : Type u
inst✝² : Group G
F : Type v
inst✝¹ : Field F
inst✝ : MulSemiringAction G F
s : Finset F
⊢ IsEmpty ↑↑∅

Tactic:
simp

Result:
no goals

Proof state:
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 : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g : G
⊢ g • l i = l i

Tactic:
refine
  eq_of_sub_eq_zero
    (linearIndependent_iff'.1 (ih hs.1) s.attach (fun i => g • l i - l i) ?_ ⟨i, his⟩
        (mem_attach _ _) :
      _)

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 : ∑ i ∈ s, l i • (toFun G F) i = (toFun G F) a
i : F
his : i ∈ s
g : G
⊢ ∑ i ∈ s.attach, (fun i => g • l ↑i - l ↑i) i • (toFun G F) ↑i = 0