-- Mathlib/LinearAlgebra/LinearIndependent/Basic.lean
theorem linearIndependent_sum {v : ι ⊕ ι' → M} :
    LinearIndependent R v ↔
      LinearIndependent R (v ∘ Sum.inl) ∧
        LinearIndependent R (v ∘ Sum.inr) ∧
          Disjoint (Submodule.span R (range (v ∘ Sum.inl)))
            (Submodule.span R (range (v ∘ Sum.inr)))

Proof state:
case refine_2.intro.intro.inl
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : ∀ x ∈ span R (v '' range Sum.inl), ∀ y ∈ span R (v '' range Sum.inr), x = y → x = 0
s : Finset (ι ⊕ ι')
g : ι ⊕ ι' → R
hg : ∑ i ∈ s, g i • v i = 0
this :
  ∑ i ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl i) =
    -∑ i ∈ s.preimage Sum.inr ⋯, (fun x => g x • v x) (Sum.inr i)
A : ∑ c ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl c) = 0
i : ι
hi : Sum.inl i ∈ s
⊢ g (Sum.inl i) = 0

Tactic:
exact hl _ _ A i (Finset.mem_preimage.2 hi)

Result:
no goals

Proof state:
case refine_1
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
⊢ LinearIndependent R v →
    LinearIndependent R (v ∘ Sum.inl) ∧
      LinearIndependent R (v ∘ Sum.inr) ∧ Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))

Tactic:
intro h

Result:
case refine_1
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
h : LinearIndependent R v
⊢ LinearIndependent R (v ∘ Sum.inl) ∧
    LinearIndependent R (v ∘ Sum.inr) ∧ Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))

Proof state:
case refine_2.intro.intro
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : LinearIndependent R (v ∘ Sum.inl)
hr : LinearIndependent R (v ∘ Sum.inr)
hlr : Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))
⊢ LinearIndependent R v

Tactic:
rw [linearIndependent_iff'] at *

Result:
case refine_2.intro.intro
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))
⊢ ∀ (s : Finset (ι ⊕ ι')) (g : ι ⊕ ι' → R), ∑ i ∈ s, g i • v i = 0 → ∀ i ∈ s, g i = 0

Proof state:
case refine_2
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
⊢ LinearIndependent R (v ∘ Sum.inl) ∧
      LinearIndependent R (v ∘ Sum.inr) ∧ Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr)) →
    LinearIndependent R v

Tactic:
rintro ⟨hl, hr, hlr⟩

Result:
case refine_2.intro.intro
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : LinearIndependent R (v ∘ Sum.inl)
hr : LinearIndependent R (v ∘ Sum.inr)
hlr : Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))
⊢ LinearIndependent R v

Proof state:
case refine_2.intro.intro
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))
s : Finset (ι ⊕ ι')
g : ι ⊕ ι' → R
hg : ∑ i ∈ s, g i • v i = 0
i : ι ⊕ ι'
hi : i ∈ s
this :
  ∑ i ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl i) =
    -∑ i ∈ s.preimage Sum.inr ⋯, (fun x => g x • v x) (Sum.inr i)
⊢ g i = 0

Tactic:
rw [disjoint_def'] at hlr

Result:
case refine_2.intro.intro
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : ∀ x ∈ span R (v '' range Sum.inl), ∀ y ∈ span R (v '' range Sum.inr), x = y → x = 0
s : Finset (ι ⊕ ι')
g : ι ⊕ ι' → R
hg : ∑ i ∈ s, g i • v i = 0
i : ι ⊕ ι'
hi : i ∈ s
this :
  ∑ i ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl i) =
    -∑ i ∈ s.preimage Sum.inr ⋯, (fun x => g x • v x) (Sum.inr i)
⊢ g i = 0

Proof state:
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : ∀ x ∈ span R (v '' range Sum.inl), ∀ y ∈ span R (v '' range Sum.inr), x = y → x = 0
s : Finset (ι ⊕ ι')
g : ι ⊕ ι' → R
hg : ∑ i ∈ s, g i • v i = 0
i : ι ⊕ ι'
hi : i ∈ s
this :
  ∑ i ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl i) =
    -∑ i ∈ s.preimage Sum.inr ⋯, (fun x => g x • v x) (Sum.inr i)
⊢ ?m.165290

Tactic:
refine hlr _ (sum_mem fun i _ => ?_) _ (neg_mem <| sum_mem fun i _ => ?_) this

Result:
case refine_1
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : ∀ x ∈ span R (v '' range Sum.inl), ∀ y ∈ span R (v '' range Sum.inr), x = y → x = 0
s : Finset (ι ⊕ ι')
g : ι ⊕ ι' → R
hg : ∑ i ∈ s, g i • v i = 0
i✝ : ι ⊕ ι'
hi : i✝ ∈ s
this :
  ∑ i ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl i) =
    -∑ i ∈ s.preimage Sum.inr ⋯, (fun x => g x • v x) (Sum.inr i)
i : ι
x✝ : i ∈ s.preimage Sum.inl ⋯
⊢ (fun x => g x • v x) (Sum.inl i) ∈ span R (v '' range Sum.inl)

case refine_2
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
hl : ∀ (s : Finset ι) (g : ι → R), ∑ i ∈ s, g i • (v ∘ Sum.inl) i = 0 → ∀ i ∈ s, g i = 0
hr : ∀ (s : Finset ι') (g : ι' → R), ∑ i ∈ s, g i • (v ∘ Sum.inr) i = 0 → ∀ i ∈ s, g i = 0
hlr : ∀ x ∈ span R (v '' range Sum.inl), ∀ y ∈ span R (v '' range Sum.inr), x = y → x = 0
s : Finset (ι ⊕ ι')
g : ι ⊕ ι' → R
hg : ∑ i ∈ s, g i • v i = 0
i✝ : ι ⊕ ι'
hi : i✝ ∈ s
this :
  ∑ i ∈ s.preimage Sum.inl ⋯, (fun x => g x • v x) (Sum.inl i) =
    -∑ i ∈ s.preimage Sum.inr ⋯, (fun x => g x • v x) (Sum.inr i)
i : ι'
x✝ : i ∈ s.preimage Sum.inr ⋯
⊢ (fun x => g x • v x) (Sum.inr i) ∈ span R (v '' range Sum.inr)

Proof state:
case refine_1
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
h : LinearIndependent R v
⊢ Disjoint (span R (v '' range Sum.inl)) (span R (v '' range Sum.inr))

Tactic:
refine h.disjoint_span_image ?_

Result:
case refine_1
ι : Type u'
ι' : Type u_1
R : Type u_2
M : Type u_4
inst✝² : Ring R
inst✝¹ : AddCommGroup M
inst✝ : Module R M
v : ι ⊕ ι' → M
h : LinearIndependent R v
⊢ Disjoint (range Sum.inl) (range Sum.inr)