PhysLean/HepLean/PerturbationTheory/Koszul/KoszulSign.lean

/-
Copyright (c) 2024 Joseph Tooby-Smith. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Joseph Tooby-Smith
-/
import HepLean.PerturbationTheory.Koszul.KoszulSignInsert
/-!

# Koszul sign

-/

namespace Wick

open HepLean.List
open FieldStatistic

variable {𝓕 : Type} (q : 𝓕 → FieldStatistic) (le : 𝓕 → 𝓕 → Prop) [DecidableRel le]

/-- Gives a factor of `- 1` for every fermion-fermion (`q` is `1`) crossing that occurs when sorting
  a list of based on `r`. -/
def koszulSign (q : 𝓕 → FieldStatistic) (le : 𝓕 → 𝓕 → Prop) [DecidableRel le] :
    List 𝓕 → ℂ
  | [] => 1
  | a :: l => koszulSignInsert q le a l * koszulSign q le l

@[simp]
lemma koszulSign_singleton (q : 𝓕 → FieldStatistic) (le : 𝓕 → 𝓕 → Prop) [DecidableRel le] (φ : 𝓕) :
    koszulSign q le [φ] = 1 := by
  simp [koszulSign, koszulSignInsert]

lemma koszulSign_mul_self (l : List 𝓕) : koszulSign q le l * koszulSign q le l = 1 := by
  induction l with
  | nil => simp [koszulSign]
  | cons a l ih =>
    simp only [koszulSign]
    trans (koszulSignInsert q le a l * koszulSignInsert q le a l) *
      (koszulSign q le l * koszulSign q le l)
    · ring
    · rw [ih, koszulSignInsert_mul_self, mul_one]

@[simp]
lemma koszulSign_freeMonoid_of (φ : 𝓕) : koszulSign q le (FreeMonoid.of φ) = 1 := by
  simp only [koszulSign, mul_one]
  rfl

lemma koszulSignInsert_erase_boson {𝓕 : Type} (q : 𝓕 → FieldStatistic)
    (le : 𝓕 → 𝓕 → Prop) [DecidableRel le] (φ : 𝓕) :
    (φs : List 𝓕) → (n : Fin φs.length) → (heq : q (φs.get n) = bosonic) →
    koszulSignInsert q le φ (φs.eraseIdx n) = koszulSignInsert q le φ φs
  | [], _, _ => by
    simp
  | r1 :: r, ⟨0, h⟩, hr => by
    simp only [List.eraseIdx_zero, List.tail_cons]
    simp only [List.length_cons, Fin.zero_eta, List.get_eq_getElem, Fin.val_zero,
      List.getElem_cons_zero] at hr
    rw [koszulSignInsert]
    simp [hr]
  | r1 :: r, ⟨n + 1, h⟩, hr => by
    simp only [List.eraseIdx_cons_succ]
    rw [koszulSignInsert, koszulSignInsert]
    rw [koszulSignInsert_erase_boson q le φ r ⟨n, Nat.succ_lt_succ_iff.mp h⟩ hr]

lemma koszulSign_erase_boson {𝓕 : Type} (q : 𝓕 → FieldStatistic) (le : 𝓕 → 𝓕 → Prop)
    [DecidableRel le] :
    (φs : List 𝓕) → (n : Fin φs.length) → (heq : q (φs.get n) = bosonic) →
    koszulSign q le (φs.eraseIdx n) = koszulSign q le φs
  | [], _ => by
    simp
  | φ :: φs, ⟨0, h⟩ => by
    simp only [List.length_cons, Fin.zero_eta, List.get_eq_getElem, Fin.val_zero,
      List.getElem_cons_zero, Fin.isValue, List.eraseIdx_zero, List.tail_cons, koszulSign]
    intro h
    rw [koszulSignInsert_boson]
    simp only [one_mul]
    exact h
  | φ :: φs, ⟨n + 1, h⟩ => by
    simp only [List.length_cons, List.get_eq_getElem, List.getElem_cons_succ, Fin.isValue,
      List.eraseIdx_cons_succ]
    intro h'
    rw [koszulSign, koszulSign, koszulSign_erase_boson q le φs ⟨n, Nat.succ_lt_succ_iff.mp h⟩]
    congr 1
    rw [koszulSignInsert_erase_boson q le φ φs ⟨n, Nat.succ_lt_succ_iff.mp h⟩ h']
    exact h'

lemma koszulSign_insertIdx [IsTotal 𝓕 le] [IsTrans 𝓕 le] (φ : 𝓕) :
    (φs : List 𝓕) → (n : ℕ) → (hn : n ≤ φs.length) →
    koszulSign q le (List.insertIdx n φ φs) = 𝓢(q φ, ofList q (φs.take n)) * koszulSign q le φs *
      𝓢(q φ, ofList q ((List.insertionSort le (List.insertIdx n φ φs)).take
      (insertionSortEquiv le (List.insertIdx n φ φs) ⟨n, by
        rw [List.length_insertIdx _ _]
        simp only [hn, ↓reduceIte]
        omega⟩)))
  | [], 0, h => by
    simp [koszulSign, koszulSignInsert]
  | [], n + 1, h => by
    simp at h
  | φ1 :: φs, 0, h => by
    simp only [List.insertIdx_zero, List.insertionSort, List.length_cons, Fin.zero_eta]
    rw [koszulSign]
    trans koszulSign q le (φ1 :: φs) * koszulSignInsert q le φ (φ1 :: φs)
    ring
    simp only [insertionSortEquiv, List.length_cons, Nat.succ_eq_add_one, List.insertionSort,
      orderedInsertEquiv, OrderIso.toEquiv_symm, Fin.symm_castOrderIso, HepLean.Fin.equivCons_trans,
      Equiv.trans_apply, HepLean.Fin.equivCons_zero, HepLean.Fin.finExtractOne_apply_eq,
      Fin.isValue, HepLean.Fin.finExtractOne_symm_inl_apply, RelIso.coe_fn_toEquiv,
      Fin.castOrderIso_apply, Fin.cast_mk, Fin.eta]
    conv_rhs =>
      enter [2,2, 2, 2]
      rw [orderedInsert_eq_insertIdx_orderedInsertPos]
    conv_rhs =>
      rhs
      rw [← ofList_take_insert]
      change 𝓢(q φ, ofList q ((List.insertionSort le (φ1 :: φs)).take
        (↑(orderedInsertPos le ((List.insertionSort le (φ1 :: φs))) φ))))
      rw [← koszulSignInsert_eq_exchangeSign_take q le]
    rw [ofList_take_zero]
    simp
  | φ1 :: φs, n + 1, h => by
    conv_lhs =>
      rw [List.insertIdx_succ_cons]
      rw [koszulSign]
    rw [koszulSign_insertIdx]
    conv_rhs =>
      rhs
      simp only [List.insertIdx_succ_cons]
      simp only [List.insertionSort, List.length_cons, insertionSortEquiv, Nat.succ_eq_add_one,
        Equiv.trans_apply, HepLean.Fin.equivCons_succ]
      erw [orderedInsertEquiv_fin_succ]
      simp only [Fin.eta, Fin.coe_cast]
      rhs
      simp [orderedInsert_eq_insertIdx_orderedInsertPos]
    conv_rhs =>
      lhs
      rw [ofList_take_succ_cons, map_mul, koszulSign]
    ring_nf
    conv_lhs =>
      lhs
      rw [mul_assoc, mul_comm]
    rw [mul_assoc]
    conv_rhs =>
      rw [mul_assoc, mul_assoc]
    congr 1
    let rs := (List.insertionSort le (List.insertIdx n φ φs))
    have hnsL : n < (List.insertIdx n φ φs).length := by
      rw [List.length_insertIdx _ _]
      simp only [List.length_cons, add_le_add_iff_right] at h
      simp only [h, ↓reduceIte]
      omega
    let ni : Fin rs.length := (insertionSortEquiv le (List.insertIdx n φ φs))
      ⟨n, hnsL⟩
    let nro : Fin (rs.length + 1) :=
      ⟨↑(orderedInsertPos le rs φ1), orderedInsertPos_lt_length le rs φ1⟩
    rw [koszulSignInsert_insertIdx, koszulSignInsert_cons]
    trans koszulSignInsert q le φ1 φs * (koszulSignCons q le φ1 φ *
      𝓢(q φ, ofList q (rs.take ni)))
    · simp only [rs, ni]
      ring
    trans koszulSignInsert q le φ1 φs * (𝓢(q φ, q φ1) *
          𝓢(q φ, ofList q ((List.insertIdx nro φ1 rs).take (nro.succAbove ni))))
    swap
    · simp only [rs, nro, ni]
      ring
    congr 1
    simp only [Fin.succAbove]
    have hns : rs.get ni = φ := by
      simp only [Fin.eta, rs]
      rw [← insertionSortEquiv_get]
      simp only [Function.comp_apply, Equiv.symm_apply_apply, List.get_eq_getElem, ni]
      simp_all only [List.length_cons, add_le_add_iff_right, List.getElem_insertIdx_self]
    have hc1 (hninro : ni.castSucc < nro) : ¬ le φ1 φ := by
      rw [← hns]
      exact lt_orderedInsertPos_rel le φ1 rs ni hninro
    have hc2 (hninro : ¬ ni.castSucc < nro) : le φ1 φ := by
      rw [← hns]
      refine gt_orderedInsertPos_rel le φ1 rs ?_ ni hninro
      exact List.sorted_insertionSort le (List.insertIdx n φ φs)
    by_cases hn : ni.castSucc < nro
    · simp only [hn, ↓reduceIte, Fin.coe_castSucc]
      rw [ofList_take_insertIdx_gt]
      swap
      · exact hn
      congr 1
      rw [koszulSignCons_eq_exchangeSign]
      simp only [hc1 hn, ↓reduceIte]
      rw [exchangeSign_symm]
    · simp only [hn, ↓reduceIte, Fin.val_succ]
      rw [ofList_take_insertIdx_le, map_mul, ← mul_assoc]
      congr 1
      rw [exchangeSign_mul_self, koszulSignCons]
      simp only [hc2 hn, ↓reduceIte]
      exact Nat.le_of_not_lt hn
      exact Nat.le_of_lt_succ (orderedInsertPos_lt_length le rs φ1)
    · exact Nat.le_of_lt_succ h
    · exact Nat.le_of_lt_succ h

lemma insertIdx_eraseIdx {I : Type} : (n : ℕ) → (r : List I) → (hn : n < r.length) →
    List.insertIdx n (r.get ⟨n, hn⟩) (r.eraseIdx n) = r
  | n, [], hn => by
    simp at hn
  | 0, r0 :: r, hn => by
    simp
  | n + 1, r0 :: r, hn => by
    simp only [List.length_cons, List.get_eq_getElem, List.getElem_cons_succ,
      List.eraseIdx_cons_succ, List.insertIdx_succ_cons, List.cons.injEq, true_and]
    exact insertIdx_eraseIdx n r _

lemma koszulSign_eraseIdx [IsTotal 𝓕 le] [IsTrans 𝓕 le] (φs : List 𝓕) (n : Fin φs.length) :
    koszulSign q le (φs.eraseIdx n) = koszulSign q le φs * 𝓢(q (φs.get n), ofList q (φs.take n)) *
    𝓢(q (φs.get n), ofList q (List.take (↑(insertionSortEquiv le φs n))
    (List.insertionSort le φs))) := by
  let φs' := φs.eraseIdx ↑n
  have hφs : List.insertIdx n (φs.get n) φs' = φs := by
    exact insertIdx_eraseIdx n.1 φs n.prop
  conv_rhs =>
    lhs
    lhs
    rw [← hφs]
    rw [koszulSign_insertIdx q le (φs.get n) ((φs.eraseIdx ↑n)) n (by
      rw [List.length_eraseIdx]
      simp only [Fin.is_lt, ↓reduceIte]
      omega)]
    rhs
    enter [2, 2, 2]
    rw [hφs]
  conv_rhs =>
    enter [1, 1, 2, 2, 2, 1, 1]
    rw [insertionSortEquiv_congr _ _ hφs]
  simp only [instCommGroup.eq_1, List.get_eq_getElem, Equiv.trans_apply, RelIso.coe_fn_toEquiv,
    Fin.castOrderIso_apply, Fin.cast_mk, Fin.eta, Fin.coe_cast]
  trans koszulSign q le (φs.eraseIdx ↑n) *
    (𝓢(q φs[↑n], ofList q ((φs.eraseIdx ↑n).take n)) * 𝓢(q φs[↑n], ofList q (List.take (↑n) φs))) *
    (𝓢(q φs[↑n], ofList q ((List.insertionSort le φs).take (↑((insertionSortEquiv le φs) n)))) *
    𝓢(q φs[↑n], ofList q (List.take (↑((insertionSortEquiv le φs) n)) (List.insertionSort le φs))))
  swap
  · simp only [Fin.getElem_fin]
    ring
  conv_rhs =>
    rhs
    rw [exchangeSign_mul_self]
  simp only [instCommGroup.eq_1, Fin.getElem_fin, mul_one]
  conv_rhs =>
    rhs
    rw [ofList_take_eraseIdx, exchangeSign_mul_self]
  simp

lemma koszulSign_eraseIdx_insertionSortMinPos [IsTotal 𝓕 le] [IsTrans 𝓕 le] (φ : 𝓕) (φs : List 𝓕) :
    koszulSign q le ((φ :: φs).eraseIdx (insertionSortMinPos le φ φs)) = koszulSign q le (φ :: φs)
    * 𝓢(q (insertionSortMin le φ φs), ofList q ((φ :: φs).take (insertionSortMinPos le φ φs))) := by
  rw [koszulSign_eraseIdx]
  conv_lhs =>
    rhs
    rhs
    lhs
    simp [insertionSortMinPos]
  erw [Equiv.apply_symm_apply]
  simp only [instCommGroup.eq_1, List.get_eq_getElem, List.length_cons, List.insertionSort,
    List.take_zero, ofList_empty, exchangeSign_bosonic, mul_one, mul_eq_mul_left_iff]
  apply Or.inl
  rfl

end Wick