feat: Start permutation contraction comm
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245
HepLean/Mathematics/Fin.lean
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HepLean/Mathematics/Fin.lean
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/-
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Copyright (c) 2024 Joseph Tooby-Smith. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Joseph Tooby-Smith
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-/
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import Mathlib.LinearAlgebra.PiTensorProduct
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import Mathlib.Tactic.Polyrith
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import Mathlib.Tactic.Linarith
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/-!
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# Fin lemmas
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The purpose of this file is to define some results Fin currently
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in Mathlib.
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At some point these should either be up-streamed to Mathlib or replaced with definitions already
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in Mathlib.
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-/
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namespace HepLean.Fin
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open Fin
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variable {n : Nat}
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def predAboveI (i x : Fin n.succ.succ) : Fin n.succ :=
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if h : x.val < i.val then
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⟨x.val, by omega⟩
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else
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⟨x.val - 1, by
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by_cases hx : x = 0
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· omega
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· omega⟩
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lemma predAboveI_self (i : Fin n.succ.succ) : predAboveI i i = ⟨i.val - 1, by omega⟩ := by
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simp [predAboveI]
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@[simp]
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lemma predAboveI_succAbove (i : Fin n.succ.succ) (x : Fin n.succ) :
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predAboveI i (Fin.succAbove i x) = x := by
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simp [predAboveI, Fin.succAbove, Fin.ext_iff]
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split_ifs
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· rfl
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· rename_i h1 h2
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simp [Fin.lt_def] at h1 h2
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omega
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· rfl
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lemma succsAbove_predAboveI {i x : Fin n.succ.succ} (h : i ≠ x) :
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Fin.succAbove i (predAboveI i x) = x := by
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simp [Fin.succAbove, predAboveI, Fin.ext_iff]
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split_ifs
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· rfl
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· rename_i h1 h2
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rw [Fin.lt_def] at h1 h2
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simp
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simp at h2
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rw [Fin.le_def] at h2
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omega
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· rename_i h1 h2
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simp at h1
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rw [Fin.le_def] at h1
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rw [Fin.lt_def] at h2
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simp at h2
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omega
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· rename_i h1 h2
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simp
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simp at h1
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rw [Fin.le_def] at h1
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omega
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lemma predAbove_eq_iff {i x : Fin n.succ.succ} (h : i ≠ x) (y : Fin n.succ) :
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y = predAboveI i x ↔ i.succAbove y = x := by
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apply Iff.intro
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· intro h
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subst h
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rw [succsAbove_predAboveI h]
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· intro h
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rw [← h]
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simp
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lemma predAboveI_lt {i x : Fin n.succ.succ} (h : x.val < i.val) :
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predAboveI i x = ⟨x.val, by omega⟩ := by
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simp [predAboveI, h]
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lemma predAboveI_ge {i x : Fin n.succ.succ} (h : i.val < x.val) :
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predAboveI i x = ⟨x.val - 1, by omega⟩ := by
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simp [predAboveI, h]
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omega
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/-- The equivalence between `Fin n.succ` and `Fin 1 ⊕ Fin n` extracting the
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`i`th component. -/
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def finExtractOne {n : ℕ} (i : Fin n.succ) : Fin n.succ ≃ Fin 1 ⊕ Fin n :=
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(finCongr (by omega : n.succ = i + 1 + (n - i))).trans <|
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finSumFinEquiv.symm.trans <|
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(Equiv.sumCongr (finSumFinEquiv.symm.trans (Equiv.sumComm (Fin i) (Fin 1)))
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(Equiv.refl (Fin (n-i)))).trans <|
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(Equiv.sumAssoc (Fin 1) (Fin i) (Fin (n - i))).trans <|
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Equiv.sumCongr (Equiv.refl (Fin 1)) (finSumFinEquiv.trans (finCongr (by omega)))
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lemma finExtractOne_apply_eq {n : ℕ} (i : Fin n.succ) :
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finExtractOne i i = Sum.inl 0 := by
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simp [finExtractOne]
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have h1 : Fin.cast (finExtractOne.proof_1 i) i = Fin.castAdd ((n - ↑i) ) ⟨i.1, lt_add_one i.1⟩ := by
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simp [Fin.ext_iff]
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rw [h1, finSumFinEquiv_symm_apply_castAdd]
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simp
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have h2 : @Fin.mk (↑i + 1) ↑i (lt_add_one i.1) = Fin.natAdd i.val 1 := by
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simp [Fin.ext_iff]
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rw [h2, finSumFinEquiv_symm_apply_natAdd]
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rfl
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lemma finExtractOne_symm_inr {n : ℕ} (i : Fin n.succ) :
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(finExtractOne i).symm ∘ Sum.inr = i.succAbove := by
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ext x
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simp only [Nat.succ_eq_add_one, finExtractOne, Function.comp_apply, Equiv.symm_trans_apply,
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finCongr_symm, Equiv.symm_symm, Equiv.sumCongr_symm, Equiv.refl_symm, Equiv.sumCongr_apply,
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Equiv.coe_refl, Sum.map_inr, finCongr_apply, Fin.coe_cast]
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change (finSumFinEquiv
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(Sum.map (⇑(finSumFinEquiv.symm.trans (Equiv.sumComm (Fin ↑i) (Fin 1))).symm) id
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((Equiv.sumAssoc (Fin 1) (Fin ↑i) (Fin (n - i))).symm
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(Sum.inr (finSumFinEquiv.symm (Fin.cast (finExtractOne.proof_2 i).symm x)))))).val = _
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by_cases hi : x.1 < i.1
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· have h1 : (finSumFinEquiv.symm (Fin.cast (finExtractOne.proof_2 i).symm x)) =
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Sum.inl ⟨x, hi⟩ := by
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rw [← finSumFinEquiv_symm_apply_castAdd]
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apply congrArg
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ext
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simp
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rw [h1]
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simp only [Nat.succ_eq_add_one, Equiv.sumAssoc_symm_apply_inr_inl, Sum.map_inl,
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Equiv.symm_trans_apply, Equiv.symm_symm, Equiv.sumComm_symm, Equiv.sumComm_apply,
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Sum.swap_inr, finSumFinEquiv_apply_left, Fin.castAdd_mk]
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rw [Fin.succAbove]
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split
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· rfl
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rename_i hn
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simp_all only [Nat.succ_eq_add_one, not_lt, Fin.le_def, Fin.coe_castSucc, Fin.val_succ,
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self_eq_add_right, one_ne_zero]
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omega
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· have h1 : (finSumFinEquiv.symm (Fin.cast (finExtractOne.proof_2 i).symm x)) =
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Sum.inr ⟨x - i, by omega⟩ := by
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rw [← finSumFinEquiv_symm_apply_natAdd]
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apply congrArg
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ext
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simp only [Nat.succ_eq_add_one, Fin.coe_cast, Fin.natAdd_mk]
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omega
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rw [h1, Fin.succAbove]
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split
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· rename_i hn
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simp_all [Fin.lt_def]
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simp only [Nat.succ_eq_add_one, Equiv.sumAssoc_symm_apply_inr_inr, Sum.map_inr, id_eq,
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finSumFinEquiv_apply_right, Fin.natAdd_mk, Fin.val_succ]
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omega
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@[simp]
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lemma finExtractOne_symm_inr_apply {n : ℕ} (i : Fin n.succ) (x : Fin n) :
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(finExtractOne i).symm (Sum.inr x) = i.succAbove x := calc
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_ = ((finExtractOne i).symm ∘ Sum.inr) x := rfl
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_ = i.succAbove x := by rw [finExtractOne_symm_inr]
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@[simp]
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lemma finExtractOne_symm_inl_apply {n : ℕ} (i : Fin n.succ) :
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(finExtractOne i).symm (Sum.inl 0) = i := by
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simp only [Nat.succ_eq_add_one, finExtractOne, Fin.isValue, Equiv.symm_trans_apply, finCongr_symm,
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Equiv.symm_symm, Equiv.sumCongr_symm, Equiv.refl_symm, Equiv.sumCongr_apply, Equiv.coe_refl,
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Sum.map_inl, id_eq, Equiv.sumAssoc_symm_apply_inl, Equiv.sumComm_symm, Equiv.sumComm_apply,
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Sum.swap_inl, finSumFinEquiv_apply_right, finSumFinEquiv_apply_left, finCongr_apply]
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ext
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rfl
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def finExtractOnPermHom (i : Fin n.succ.succ) (σ : Fin n.succ.succ ≃ Fin n.succ.succ) :
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Fin n.succ → Fin n.succ := fun x => predAboveI (σ i) (σ ((finExtractOne i).symm (Sum.inr x)))
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lemma finExtractOnPermHom_inv (i : Fin n.succ.succ) (σ : Fin n.succ.succ ≃ Fin n.succ.succ) :
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(finExtractOnPermHom (σ i) σ.symm) ∘ (finExtractOnPermHom i σ) = id := by
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funext x
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simp [finExtractOnPermHom]
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by_cases h : σ (i.succAbove x) < σ i
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· rw [predAboveI_lt h, Fin.succAbove_of_castSucc_lt]
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· simp
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· simp_all [Fin.lt_def]
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have hσ : σ (i.succAbove x) ≠ σ i := by
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simp only [Nat.succ_eq_add_one, ne_eq, EmbeddingLike.apply_eq_iff_eq]
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exact Fin.succAbove_ne i x
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have hn : σ i < σ (i.succAbove x) := by omega
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rw [predAboveI_ge hn]
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rw [Fin.succAbove_of_le_castSucc]
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· simp
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trans predAboveI i (σ.symm (σ (i.succAbove x)))
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· congr
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exact Nat.sub_add_cancel (Fin.lt_of_le_of_lt (Fin.zero_le (σ i)) hn)
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simp
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rw [Fin.le_def]
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simp
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omega
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def finExtractOnePerm (i : Fin n.succ.succ) (σ : Fin n.succ.succ ≃ Fin n.succ.succ) :
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Fin n.succ ≃ Fin n.succ where
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toFun x := finExtractOnPermHom i σ x
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invFun x := finExtractOnPermHom (σ i) σ.symm x
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left_inv x := by
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simpa using congrFun (finExtractOnPermHom_inv i σ) x
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right_inv x := by
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simpa using congrFun (finExtractOnPermHom_inv (σ i) σ.symm) x
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/-- The equivalence of types `Fin n.succ.succ ≃ (Fin 1 ⊕ Fin 1) ⊕ Fin n` extracting
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the `i` and `(i.succAbove j)`. -/
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def finExtractTwo {n : ℕ} (i : Fin n.succ.succ) (j : Fin n.succ) :
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Fin n.succ.succ ≃ (Fin 1 ⊕ Fin 1) ⊕ Fin n :=
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(finExtractOne i).trans <|
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(Equiv.sumCongr (Equiv.refl (Fin 1)) (finExtractOne j)).trans <|
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(Equiv.sumAssoc (Fin 1) (Fin 1) (Fin n)).symm
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@[simp]
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lemma finExtractTwo_apply_fst {n : ℕ} (i : Fin n.succ.succ) (j : Fin n.succ) :
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finExtractTwo i j i = Sum.inl (Sum.inl 0) := by
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simp [finExtractTwo]
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simp [finExtractOne_apply_eq]
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lemma finExtractTwo_symm_inr {n : ℕ} (i : Fin n.succ.succ) (j : Fin n.succ) :
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(finExtractTwo i j).symm ∘ Sum.inr = i.succAbove ∘ j.succAbove := by
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rw [finExtractTwo]
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ext1 x
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simp
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@[simp]
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lemma finExtractTwo_symm_inr_apply {n : ℕ} (i : Fin n.succ.succ) (j : Fin n.succ) (x : Fin n) :
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(finExtractTwo i j).symm (Sum.inr x) = i.succAbove (j.succAbove x) := by
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rw [finExtractTwo]
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simp
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@[simp]
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lemma finExtractTwo_symm_inl_inr_apply {n : ℕ} (i : Fin n.succ.succ) (j : Fin n.succ) :
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(finExtractTwo i j).symm (Sum.inl (Sum.inr 0)) = i.succAbove j := by
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rw [finExtractTwo]
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simp
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@[simp]
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lemma finExtractTwo_symm_inl_inl_apply {n : ℕ} (i : Fin n.succ.succ) (j : Fin n.succ) :
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(finExtractTwo i j).symm (Sum.inl (Sum.inl 0)) = i := by
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rw [finExtractTwo]
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simp
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end HepLean.Fin
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