Module: Mathlib.LinearAlgebra.SymplecticGroup

Goal state before:
pp: l : Type u_1
inst✝² : DecidableEq l
inst✝¹ : Fintype l
R : Type u_3
inst✝ : Field R
A C : Matrix l l R
hker : ∀ (x : l → R), A • x = 0 → C • x = 0 → x = 0
hsymm : Aᵀ * C = Cᵀ * A
V U : Matrix l l R
s : l ≃ Fin C.rank ⊕ Fin (Fintype.card l - C.rank)
hV : IsUnit V
hU : IsUnit U
P : Matrix l l R := V * C * U
heq : P = (fromBlocks 1 0 0 0).submatrix ⇑s ⇑s
P_def : P = V * C * U
Q : Matrix l l R := Vᵀ⁻¹ * A * U
Q_def : Q = Vᵀ⁻¹ * A * U
f : Matrix l l R → Matrix (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) R :=
  fun x ↦ x.submatrix ⇑s.symm ⇑s.symm
hf : ∀ (x : Matrix l l R), f x = x.submatrix ⇑s.symm ⇑s.symm
f_unit : ∀ {x : Matrix l l R}, IsUnit x → IsUnit (f x)
f_mul : ∀ (x y : Matrix l l R), f (x * y) = f x * f y
x✝² : Invertible V
x✝¹ : Invertible U
x✝ : Invertible (f Vᵀ)
con1 : ∀ (x : Fin C.rank ⊕ Fin (Fintype.card l - C.rank) → R), f Q • x = 0 → f P • x = 0 → x = 0
con2 : (f Q).toBlocks₁₁ᵀ = (f Q).toBlocks₁₁ ∧ (f Q).toBlocks₁₂ = 0
con3 : IsUnit (f Q).toBlocks₂₂
Y : Matrix (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) R :=
  fromBlocks (1 - (f Q).toBlocks₁₁) 0 0 0
Y_def : Y = fromBlocks (1 - (f Q).toBlocks₁₁) 0 0 0
hY_symm : Y.IsSymm
X : Matrix (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) R := f Vᵀ * Y * f V
X_def : X = f Vᵀ * Y * f V
⊢ (A + X.submatrix ⇑s ⇑s * C).submatrix ⇑s.symm ⇑s.symm =
    Vᵀ.submatrix ⇑s.symm ⇑s.symm * (Q.submatrix ⇑s.symm ⇑s.symm + Y * P.submatrix ⇑s.symm ⇑s.symm) *
      U⁻¹.submatrix ⇑s.symm ⇑s.symm

Tactic:
submatrix_add,

Goal state after:
l : Type u_1
inst✝² : DecidableEq l
inst✝¹ : Fintype l
R : Type u_3
inst✝ : Field R
A C : Matrix l l R
hker : ∀ (x : l → R), A • x = 0 → C • x = 0 → x = 0
hsymm : Aᵀ * C = Cᵀ * A
V U : Matrix l l R
s : l ≃ Fin C.rank ⊕ Fin (Fintype.card l - C.rank)
hV : IsUnit V
hU : IsUnit U
P : Matrix l l R := V * C * U
heq : P = (fromBlocks 1 0 0 0).submatrix ⇑s ⇑s
P_def : P = V * C * U
Q : Matrix l l R := Vᵀ⁻¹ * A * U
Q_def : Q = Vᵀ⁻¹ * A * U
f : Matrix l l R → Matrix (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) R :=
  fun x ↦ x.submatrix ⇑s.symm ⇑s.symm
hf : ∀ (x : Matrix l l R), f x = x.submatrix ⇑s.symm ⇑s.symm
f_unit : ∀ {x : Matrix l l R}, IsUnit x → IsUnit (f x)
f_mul : ∀ (x y : Matrix l l R), f (x * y) = f x * f y
x✝² : Invertible V
x✝¹ : Invertible U
x✝ : Invertible (f Vᵀ)
con1 : ∀ (x : Fin C.rank ⊕ Fin (Fintype.card l - C.rank) → R), f Q • x = 0 → f P • x = 0 → x = 0
con2 : (f Q).toBlocks₁₁ᵀ = (f Q).toBlocks₁₁ ∧ (f Q).toBlocks₁₂ = 0
con3 : IsUnit (f Q).toBlocks₂₂
Y : Matrix (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) R :=
  fromBlocks (1 - (f Q).toBlocks₁₁) 0 0 0
Y_def : Y = fromBlocks (1 - (f Q).toBlocks₁₁) 0 0 0
hY_symm : Y.IsSymm
X : Matrix (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) (Fin C.rank ⊕ Fin (Fintype.card l - C.rank)) R := f Vᵀ * Y * f V
X_def : X = f Vᵀ * Y * f V
⊢ (A.submatrix + (X.submatrix ⇑s ⇑s * C).submatrix) ⇑s.symm ⇑s.symm =
    Vᵀ.submatrix ⇑s.symm ⇑s.symm * (Q.submatrix ⇑s.symm ⇑s.symm + Y * P.submatrix ⇑s.symm ⇑s.symm) *
      U⁻¹.submatrix ⇑s.symm ⇑s.symm