PhysLean/HepLean/Tensors/Tree/NodeIdentities/Basic.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.Tensors.Tree.Basic
/-!

## Basic node identities

This file contains the basic node identities for tensor trees.
More compliciated identities appear in there own files.

-/

open IndexNotation
open CategoryTheory
open MonoidalCategory
open OverColor
open HepLean.Fin
open TensorProduct

namespace TensorTree

variable {S : TensorSpecies}

/-!

## Equality of constructors.

-/

informal_lemma constVecNode_eq_vecNode where
  math :≈ "A constVecNode has equal tensor to the vecNode with the map evaluated at 1."
  deps :≈ [``constVecNode, ``vecNode]

informal_lemma constTwoNode_eq_twoNode where
  math :≈ "A constTwoNode has equal tensor to the twoNode with the map evaluated at 1."
  deps :≈ [``constTwoNode, ``twoNode]

/-!

## Negation

-/

@[simp]
lemma neg_neg (t : TensorTree S c) : (neg (neg t)).tensor = t.tensor := by
  simp only [neg_tensor, _root_.neg_neg]

@[simp]
lemma neg_fst_prod {c1 : Fin n → S.C} {c2 : Fin m → S.C} (T1 : TensorTree S c1)
    (T2 : TensorTree S c2) :
    (prod (neg T1) T2).tensor = (neg (prod T1 T2)).tensor := by
  simp only [prod_tensor, Functor.id_obj, Action.instMonoidalCategory_tensorObj_V,
    Equivalence.symm_inverse, Action.functorCategoryEquivalence_functor,
    Action.FunctorCategoryEquivalence.functor_obj_obj, neg_tensor, neg_tmul, map_neg]

@[simp]
lemma neg_snd_prod {c1 : Fin n → S.C} {c2 : Fin m → S.C} (T1 : TensorTree S c1)
    (T2 : TensorTree S c2) :
    (prod T1 (neg T2)).tensor = (neg (prod T1 T2)).tensor := by
  simp only [prod_tensor, Functor.id_obj, Action.instMonoidalCategory_tensorObj_V,
    Equivalence.symm_inverse, Action.functorCategoryEquivalence_functor,
    Action.FunctorCategoryEquivalence.functor_obj_obj, neg_tensor, tmul_neg, map_neg]

@[simp]
lemma neg_contr {n : ℕ} {c : Fin n.succ.succ → S.C} {i : Fin n.succ.succ} {j : Fin n.succ}
    {h : c (i.succAbove j) = S.τ (c i)}
    (t : TensorTree S c) : (contr i j h (neg t)).tensor = (neg (contr i j h t)).tensor := by
  simp only [Nat.succ_eq_add_one, contr_tensor, neg_tensor, map_neg]

lemma neg_perm {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (t : TensorTree S c) :
    (perm σ (neg t)).tensor = (neg (perm σ t)).tensor := by
  simp only [perm_tensor, neg_tensor, map_neg]

@[simp]
lemma neg_add (t : TensorTree S c) : (add (neg t) t).tensor = 0 := by
  rw [add_tensor, neg_tensor]
  simp only [neg_add_cancel]

@[simp]
lemma add_neg (t : TensorTree S c) : (add t (neg t)).tensor = 0 := by
  rw [add_tensor, neg_tensor]
  simp only [add_neg_cancel]

/-!

## Basic perm identities

-/

/-- Applying two permutations is the same as applying the transitive permutation. -/
lemma perm_perm {n : ℕ} {c : Fin n → S.C} {c1 : Fin n → S.C} {c2 : Fin n → S.C}
    (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (σ2 : (OverColor.mk c1) ⟶ (OverColor.mk c2))
    (t : TensorTree S c) : (perm σ2 (perm σ t)).tensor = (perm (σ ≫ σ2) t).tensor := by
  simp [perm_tensor]

/-- Applying the identity permutation is the same as not applying a permutation. -/
lemma perm_id (t : TensorTree S c) : (perm (𝟙 (OverColor.mk c)) t).tensor = t.tensor := by
  simp [perm_tensor]

/-- Applying a permutation which is equal to the identity permutation is the same
  as not applying a permutation. -/
lemma perm_eq_id {n : ℕ} {c : Fin n → S.C} (σ : (OverColor.mk c) ⟶ (OverColor.mk c))
    (h : σ = 𝟙 _) (t : TensorTree S c) : (perm σ t).tensor = t.tensor := by
  simp [perm_tensor, h]

lemma perm_eq_of_eq_perm {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (σ : (OverColor.mk c) ≅ (OverColor.mk c1))
    {t : TensorTree S c} {t2 : TensorTree S c1} (h : (perm σ.hom t).tensor = t2.tensor) :
    t.tensor = (perm σ.inv t2).tensor := by
  rw [perm_tensor, ← h]
  change _ = (S.F.map σ.hom ≫ S.F.map σ.inv).hom _
  simp only [Iso.map_hom_inv_id, Action.id_hom, ModuleCat.id_apply]

lemma perm_eq_iff_eq_perm {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (σ : (OverColor.mk c) ⟶  (OverColor.mk c1))
    {t : TensorTree S c} {t2 : TensorTree S c1} :
    (perm σ t).tensor = t2.tensor ↔ t.tensor =
    (perm (equivToHomEq (Hom.toEquiv σ).symm (fun x => Hom.toEquiv_comp_apply σ x)) t2).tensor := by
  refine Iff.intro (fun h => ?_)  (fun h => ?_)
  · simp [perm_tensor, ← h]
    change _ = (S.F.map _ ≫ S.F.map _).hom _
    rw [← S.F.map_comp]
    have h1 : (σ ≫ equivToHomEq (Hom.toEquiv σ).symm (fun x => Hom.toEquiv_comp_apply σ x)) = 𝟙 _ := by
      apply Hom.ext
      ext x
      change (Hom.toEquiv σ).symm ((Hom.toEquiv σ) x) = x
      simp
    rw [h1]
    simp
  · rw [perm_tensor, h]
    change (S.F.map _ ≫ S.F.map _).hom _ = _
    rw [← S.F.map_comp]
    have h1 : (equivToHomEq (Hom.toEquiv σ).symm (fun x => Hom.toEquiv_comp_apply σ x) ≫ σ) = 𝟙 _ := by
      apply Hom.ext
      ext x
      change (Hom.toEquiv σ) ((Hom.toEquiv σ).symm x) = x
      simp
    rw [h1]
    simp

/-!

## Vector based identities

These identities are related to the fact that all the maps are linear.

-/

lemma smul_smul (t : TensorTree S c) (a b : S.k) :
    (smul a (smul b t)).tensor = (smul (a * b) t).tensor := by
  simp only [smul_tensor]
  exact _root_.smul_smul a b t.tensor

lemma smul_one (t : TensorTree S c) :
    (smul 1 t).tensor = t.tensor := by
  simp [smul_tensor]

lemma smul_eq_one (t : TensorTree S c) (a : S.k) (h : a = 1) :
    (smul a t).tensor = t.tensor := by
  rw [h]
  exact smul_one _

/-- The addition node is commutative. -/
lemma add_comm (t1 t2 : TensorTree S c) : (add t1 t2).tensor = (add t2 t1).tensor := by
  simp only [add_tensor]
  exact AddCommMagma.add_comm t1.tensor t2.tensor

/-- The addition node is associative. -/
lemma add_assoc (t1 t2 t3 : TensorTree S c) :
    (add (add t1 t2) t3).tensor = (add t1 (add t2 t3)).tensor := by
  simp only [add_tensor]
  exact _root_.add_assoc t1.tensor t2.tensor t3.tensor

/-- When the same permutation acts on both arguments of an addition, the permutation
  can be moved out of the addition. -/
lemma add_perm {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (t t1 : TensorTree S c) :
    (add (perm σ t) (perm σ t1)).tensor = (perm σ (add t t1)).tensor := by
  simp only [add_tensor, perm_tensor, map_add]

lemma perm_add {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (t t1 : TensorTree S c) :
    (perm σ (add t t1)).tensor = (add (perm σ t) (perm σ t1)).tensor := by
  simp only [add_tensor, perm_tensor, map_add]

lemma perm_smul {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (σ : (OverColor.mk c) ⟶ (OverColor.mk c1)) (a : S.k) (t : TensorTree S c) :
    (perm σ (smul a t)).tensor = (smul a (perm σ t)).tensor := by
  simp only [smul_tensor, perm_tensor, map_smul]

/-- When the same evaluation acts on both arguments of an addition, the evaluation
  can be moved out of the addition. -/
lemma add_eval {n : ℕ} {c : Fin n.succ → S.C} (i : Fin n.succ) (e : ℕ) (t t1 : TensorTree S c) :
    (add (eval i e t) (eval i e t1)).tensor = (eval i e (add t t1)).tensor := by
  simp only [add_tensor, eval_tensor, Nat.succ_eq_add_one, map_add]

lemma contr_add {n : ℕ} {c : Fin n.succ.succ → S.C} {i : Fin n.succ.succ} {j : Fin n.succ}
    {h : c (i.succAbove j) = S.τ (c i)} (t1 t2 : TensorTree S c) :
  (contr i j h (add t1 t2)).tensor = (add (contr i j h t1) (contr i j h t2)).tensor := by
  simp [contr_tensor, add_tensor]

lemma contr_smul {n : ℕ} {c : Fin n.succ.succ → S.C} {i : Fin n.succ.succ} {j : Fin n.succ}
    {h : c (i.succAbove j) = S.τ (c i)} (a : S.k) (t : TensorTree S c) :
    (contr i j h (smul a t)).tensor = (smul a (contr i j h t)).tensor := by
  simp [contr_tensor, smul_tensor]

@[simp]
lemma add_prod {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (t1 t2 : TensorTree S c) (t3 : TensorTree S c1) :
    (prod (add t1 t2) t3).tensor = (add (prod t1 t3) (prod t2 t3)).tensor := by
  simp only [prod_tensor, Functor.id_obj, mk_hom, Action.instMonoidalCategory_tensorObj_V,
    Equivalence.symm_inverse, Action.functorCategoryEquivalence_functor,
    Action.FunctorCategoryEquivalence.functor_obj_obj, add_tensor, add_tmul, map_add]

@[simp]
lemma prod_add {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (t1 : TensorTree S c) (t2 t3 : TensorTree S c1) :
    (prod t1 (add t2 t3)).tensor = (add (prod t1 t2) (prod t1 t3)).tensor := by
  simp only [prod_tensor, Functor.id_obj, mk_hom, Action.instMonoidalCategory_tensorObj_V,
    Equivalence.symm_inverse, Action.functorCategoryEquivalence_functor,
    Action.FunctorCategoryEquivalence.functor_obj_obj, add_tensor, tmul_add, map_add]

lemma smul_prod {n m: ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (a : S.k) (t1 : TensorTree S c) (t2 : TensorTree S c1) :
    ((prod (smul a t1) t2)).tensor = (smul a (prod t1 t2)).tensor := by
  simp [prod_tensor, smul_tensor, tmul_smul, smul_tmul, map_smul]

lemma prod_smul {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (a : S.k) (t1 : TensorTree S c) (t2 : TensorTree S c1) :
    (prod t1 (smul a t2)).tensor = (smul a (prod t1 t2)).tensor := by
  simp [prod_tensor, smul_tensor, tmul_smul, smul_tmul, map_smul]

lemma prod_add_both {n m : ℕ} {c : Fin n → S.C} {c1 : Fin m → S.C}
    (t1 t2 : TensorTree S c) (t3 t4 : TensorTree S c1) :
    (prod (add t1 t2) (add t3 t4)).tensor = (add (add (prod t1 t3) (prod t1 t4))
    (add (prod t2 t3) (prod t2 t4))).tensor := by
  rw [add_prod]
  rw [add_tensor_eq_fst (prod_add _ _ _)]
  rw [add_tensor_eq_snd (prod_add _ _ _)]

end TensorTree