-- Mathlib/NumberTheory/NumberField/Units/Regulator.lean
theorem abs_det_eq_abs_det (u : Fin (rank K) → (𝓞 K)ˣ)
    {w₁ w₂ : InfinitePlace K} (e₁ : {w // w ≠ w₁} ≃ Fin (rank K))
    (e₂ : {w // w ≠ w₂} ≃ Fin (rank K)) :
    |(Matrix.of fun i w : {w // w ≠ w₁} ↦ (mult w.val : ℝ) * (w.val (u (e₁ i) : K)).log).det| =
    |(Matrix.of fun i w : {w // w ≠ w₂} ↦ (mult w.val : ℝ) * (w.val (u (e₂ i) : K)).log).det|

Proof state:
case refine_1
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
i✝ : Fin (rank K)
⊢ ∑ j : Fin (rank K + 1), Matrix.of (fun i w => ↑(f w).mult * Real.log ((f w) ((algebraMap (𝓞 K) K) ↑(u i)))) i✝ j = 0

Tactic:
simp_rw [Matrix.of_apply, ← Real.log_pow]

Result:
case refine_1
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
i✝ : Fin (rank K)
⊢ ∑ x : Fin (rank K + 1), Real.log ((f x) ((algebraMap (𝓞 K) K) ↑(u i✝)) ^ (f x).mult) = 0

Proof state:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
⊢ |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₁ i))))).det| =
    |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix
          (⇑(g.trans e₂.symm)) id).det|

Tactic:
have h := congr_arg abs <| Matrix.submatrix_succAbove_det_eq_negOnePow_submatrix_succAbove_det'
  (Matrix.of fun i w ↦ (mult (f w) : ℝ) * ((f w) (u i)).log) ?_ 0 (f.symm w₂)

Result:
case refine_2
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
h :
  |((Matrix.of fun i w => ↑(f w).mult * Real.log ((f w) ((algebraMap (𝓞 K) K) ↑(u i)))).submatrix id
          (Fin.succAbove 0)).det| =
    |(↑↑0 - ↑↑(f.symm w₂)).negOnePow •
        ((Matrix.of fun i w => ↑(f w).mult * Real.log ((f w) ((algebraMap (𝓞 K) K) ↑(u i)))).submatrix id
            (f.symm w₂).succAbove).det|
⊢ |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₁ i))))).det| =
    |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix
          (⇑(g.trans e₂.symm)) id).det|

case refine_1
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
⊢ ∀ (i : Fin (rank K)),
    ∑ j : Fin (rank K + 1), Matrix.of (fun i w => ↑(f w).mult * Real.log ((f w) ((algebraMap (𝓞 K) K) ↑(u i)))) i j = 0

Proof state:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |↑↑(Equiv.Perm.sign (g.trans e₂.symm)) *
        (Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
⊢ |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₁ i))))).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|

Tactic:
rw [abs_mul, ← Int.cast_abs, Equiv.Perm.sign_abs, Int.cast_one, one_mul] at h_col

Result:
K : Type u_1
inst✝¹ : Field K
inst✝ : NumberField K
u : Fin (rank K) → (𝓞 K)ˣ
w₁ w₂ : InfinitePlace K
e₁ : { w // w ≠ w₁ } ≃ Fin (rank K)
e₂ : { w // w ≠ w₂ } ≃ Fin (rank K)
f : Fin (rank K + 1) ≃ InfinitePlace K := (finSuccEquiv (rank K)).trans ↑((Equiv.optionSubtype w₁).symm e₁.symm)
g : { w // w ≠ w₂ } ≃ Fin (rank K) := (f.symm.subtypeEquiv ⋯).trans (finSuccAboveEquiv (f.symm w₂)).symm
h_col :
  |((Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).submatrix (⇑(g.trans e₂.symm))
          id).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|
⊢ |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₁ i))))).det| =
    |(Matrix.of fun i w => ↑(↑w).mult * Real.log (↑w ((algebraMap (𝓞 K) K) ↑(u (e₂ i))))).det|