feat: port Data.MvPolynomial.Rename (#2886)

- [x] depends on: #2861



Co-authored-by: ChrisHughes24 <chrishughes24@gmail.com>

Author: j-loreaux

Mathlib.lean

@@ -716,6 +716,7 @@ import Mathlib.Data.Multiset.Sections
 import Mathlib.Data.Multiset.Sort
 import Mathlib.Data.Multiset.Sum
 import Mathlib.Data.MvPolynomial.Basic
+import Mathlib.Data.MvPolynomial.Rename
 import Mathlib.Data.Nat.Basic
 import Mathlib.Data.Nat.Bits
 import Mathlib.Data.Nat.Bitwise

Mathlib/Data/MvPolynomial/Rename.lean

@@ -0,0 +1,349 @@
+/-
+Copyright (c) 2017 Johannes Hölzl. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Johannes Hölzl, Johan Commelin, Mario Carneiro
+
+! This file was ported from Lean 3 source module data.mv_polynomial.rename
+! leanprover-community/mathlib commit eabc6192c84ccce3936a8577a987b80b95ba75f6
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.Data.MvPolynomial.Basic
+
+/-!
+# Renaming variables of polynomials
+
+This file establishes the `rename` operation on multivariate polynomials,
+which modifies the set of variables.
+
+## Main declarations
+
+* `MvPolynomial.rename`
+* `MvPolynomial.renameEquiv`
+
+## Notation
+
+As in other polynomial files, we typically use the notation:
+
++ `σ τ α : Type*` (indexing the variables)
+
++ `R S : Type*` `[CommSemiring R]` `[CommSemiring S]` (the coefficients)
+
++ `s : σ →₀ ℕ`, a function from `σ` to `ℕ` which is zero away from a finite set.
+This will give rise to a monomial in `MvPolynomial σ R` which mathematicians might call `X^s`
+
++ `r : R` elements of the coefficient ring
+
++ `i : σ`, with corresponding monomial `X i`, often denoted `X_i` by mathematicians
+
++ `p : MvPolynomial σ α`
+
+-/
+
+
+noncomputable section
+
+open Classical BigOperators
+
+open Set Function Finsupp AddMonoidAlgebra
+
+open BigOperators
+
+variable {σ τ α R S : Type _} [CommSemiring R] [CommSemiring S]
+
+namespace MvPolynomial
+
+section Rename
+
+/-- Rename all the variables in a multivariable polynomial. -/
+def rename (f : σ → τ) : MvPolynomial σ R →ₐ[R] MvPolynomial τ R :=
+  aeval (X ∘ f)
+#align mv_polynomial.rename MvPolynomial.rename
+
+@[simp]
+theorem rename_C (f : σ → τ) (r : R) : rename f (C r) = C r :=
+  eval₂_C _ _ _
+set_option linter.uppercaseLean3 false in
+#align mv_polynomial.rename_C MvPolynomial.rename_C
+
+@[simp]
+theorem rename_X (f : σ → τ) (i : σ) : rename f (X i : MvPolynomial σ R) = X (f i) :=
+  eval₂_X _ _ _
+set_option linter.uppercaseLean3 false in
+#align mv_polynomial.rename_X MvPolynomial.rename_X
+
+theorem map_rename (f : R →+* S) (g : σ → τ) (p : MvPolynomial σ R) :
+    map f (rename g p) = rename g (map f p) := by
+  apply MvPolynomial.induction_on p
+    (fun a => by simp only [map_C, rename_C])
+    (fun p q hp hq => by simp only [hp, hq, AlgHom.map_add, RingHom.map_add]) fun p n hp => by
+    simp only [hp, rename_X, map_X, RingHom.map_mul, AlgHom.map_mul]
+#align mv_polynomial.map_rename MvPolynomial.map_rename
+
+@[simp]
+theorem rename_rename (f : σ → τ) (g : τ → α) (p : MvPolynomial σ R) :
+    rename g (rename f p) = rename (g ∘ f) p :=
+  show rename g (eval₂ C (X ∘ f) p) = _
+    by
+    simp only [rename, aeval_eq_eval₂Hom]
+    -- porting note: the Lean 3 proof of this was very fragile and included a nonterminal `simp`.
+    -- Hopefully this is less prone to breaking
+    rw [eval₂_comp_left (eval₂Hom (algebraMap R (MvPolynomial α R)) (X ∘ g)) C (X ∘ f) p]
+    simp only [(· ∘ ·), eval₂Hom_X', coe_eval₂Hom]
+    refine' eval₂Hom_congr _ rfl rfl
+    ext1; simp only [comp_apply, RingHom.coe_comp, eval₂Hom_C]
+#align mv_polynomial.rename_rename MvPolynomial.rename_rename
+
+@[simp]
+theorem rename_id (p : MvPolynomial σ R) : rename id p = p :=
+  eval₂_eta p
+#align mv_polynomial.rename_id MvPolynomial.rename_id
+
+theorem rename_monomial (f : σ → τ) (d : σ →₀ ℕ) (r : R) :
+    rename f (monomial d r) = monomial (d.mapDomain f) r := by
+  rw [rename, aeval_monomial, monomial_eq (s := Finsupp.mapDomain f d),
+    Finsupp.prod_mapDomain_index]
+  · rfl
+  · exact fun n => pow_zero _
+  · exact fun n i₁ i₂ => pow_add _ _ _
+#align mv_polynomial.rename_monomial MvPolynomial.rename_monomial
+
+theorem rename_eq (f : σ → τ) (p : MvPolynomial σ R) :
+    rename f p = Finsupp.mapDomain (Finsupp.mapDomain f) p := by
+  simp only [rename, aeval_def, eval₂, Finsupp.mapDomain, algebraMap_eq, comp_apply,
+    X_pow_eq_monomial, ← monomial_finsupp_sum_index]
+  rfl
+#align mv_polynomial.rename_eq MvPolynomial.rename_eq
+
+theorem rename_injective (f : σ → τ) (hf : Function.Injective f) :
+    Function.Injective (rename f : MvPolynomial σ R → MvPolynomial τ R) := by
+  have :
+    (rename f : MvPolynomial σ R → MvPolynomial τ R) = Finsupp.mapDomain (Finsupp.mapDomain f) :=
+    funext (rename_eq f)
+  rw [this]
+  exact Finsupp.mapDomain_injective (Finsupp.mapDomain_injective hf)
+#align mv_polynomial.rename_injective MvPolynomial.rename_injective
+
+section
+
+variable {f : σ → τ} (hf : Function.Injective f)
+
+open Classical
+
+/-- Given a function between sets of variables `f : σ → τ` that is injective with proof `hf`,
+  `MvPolynomial.killCompl hf` is the `AlgHom` from `R[τ]` to `R[σ]` that is left inverse to
+  `rename f : R[σ] → R[τ]` and sends the variables in the complement of the range of `f` to `0`. -/
+def killCompl : MvPolynomial τ R →ₐ[R] MvPolynomial σ R :=
+  aeval fun i => if h : i ∈ Set.range f then X <| (Equiv.ofInjective f hf).symm ⟨i, h⟩ else 0
+#align mv_polynomial.kill_compl MvPolynomial.killCompl
+
+theorem killCompl_comp_rename : (killCompl hf).comp (rename f) = AlgHom.id R _ :=
+  algHom_ext fun i => by
+    dsimp
+    rw [rename, killCompl, aeval_X, comp_apply, aeval_X, dif_pos, Equiv.ofInjective_symm_apply]
+#align mv_polynomial.kill_compl_comp_rename MvPolynomial.killCompl_comp_rename
+
+@[simp]
+theorem killCompl_rename_app (p : MvPolynomial σ R) : killCompl hf (rename f p) = p :=
+  AlgHom.congr_fun (killCompl_comp_rename hf) p
+#align mv_polynomial.kill_compl_rename_app MvPolynomial.killCompl_rename_app
+
+end
+
+section
+
+variable (R)
+
+/-- `MvPolynomial.rename e` is an equivalence when `e` is. -/
+@[simps apply]
+def renameEquiv (f : σ ≃ τ) : MvPolynomial σ R ≃ₐ[R] MvPolynomial τ R :=
+  { rename f with
+    toFun := rename f
+    invFun := rename f.symm
+    left_inv := fun p => by rw [rename_rename, f.symm_comp_self, rename_id]
+    right_inv := fun p => by rw [rename_rename, f.self_comp_symm, rename_id] }
+#align mv_polynomial.rename_equiv MvPolynomial.renameEquiv
+
+@[simp]
+theorem renameEquiv_refl : renameEquiv R (Equiv.refl σ) = AlgEquiv.refl :=
+  AlgEquiv.ext rename_id
+#align mv_polynomial.rename_equiv_refl MvPolynomial.renameEquiv_refl
+
+@[simp]
+theorem renameEquiv_symm (f : σ ≃ τ) : (renameEquiv R f).symm = renameEquiv R f.symm :=
+  rfl
+#align mv_polynomial.rename_equiv_symm MvPolynomial.renameEquiv_symm
+
+@[simp]
+theorem renameEquiv_trans (e : σ ≃ τ) (f : τ ≃ α) :
+    (renameEquiv R e).trans (renameEquiv R f) = renameEquiv R (e.trans f) :=
+  AlgEquiv.ext (rename_rename e f)
+#align mv_polynomial.rename_equiv_trans MvPolynomial.renameEquiv_trans
+
+end
+
+section
+
+variable (f : R →+* S) (k : σ → τ) (g : τ → S) (p : MvPolynomial σ R)
+
+theorem eval₂_rename : (rename k p).eval₂ f g = p.eval₂ f (g ∘ k) := by
+  apply MvPolynomial.induction_on p <;>
+    · intros
+      simp [*]
+#align mv_polynomial.eval₂_rename MvPolynomial.eval₂_rename
+
+theorem eval₂Hom_rename : eval₂Hom f g (rename k p) = eval₂Hom f (g ∘ k) p :=
+  eval₂_rename _ _ _ _
+#align mv_polynomial.eval₂_hom_rename MvPolynomial.eval₂Hom_rename
+
+theorem aeval_rename [Algebra R S] : aeval g (rename k p) = aeval (g ∘ k) p :=
+  eval₂Hom_rename _ _ _ _
+#align mv_polynomial.aeval_rename MvPolynomial.aeval_rename
+
+theorem rename_eval₂ (g : τ → MvPolynomial σ R) :
+    rename k (p.eval₂ C (g ∘ k)) = (rename k p).eval₂ C (rename k ∘ g) := by
+  apply MvPolynomial.induction_on p <;>
+    · intros
+      simp [*]
+#align mv_polynomial.rename_eval₂ MvPolynomial.rename_eval₂
+
+theorem rename_prodmk_eval₂ (j : τ) (g : σ → MvPolynomial σ R) :
+    rename (Prod.mk j) (p.eval₂ C g) = p.eval₂ C fun x => rename (Prod.mk j) (g x) := by
+  apply MvPolynomial.induction_on p <;>
+    · intros
+      simp [*]
+#align mv_polynomial.rename_prodmk_eval₂ MvPolynomial.rename_prodmk_eval₂
+
+theorem eval₂_rename_prod_mk (g : σ × τ → S) (i : σ) (p : MvPolynomial τ R) :
+    (rename (Prod.mk i) p).eval₂ f g = eval₂ f (fun j => g (i, j)) p := by
+  apply MvPolynomial.induction_on p <;>
+    · intros
+      simp [*]
+#align mv_polynomial.eval₂_rename_prodmk MvPolynomial.eval₂_rename_prod_mk
+
+theorem eval_rename_prod_mk (g : σ × τ → R) (i : σ) (p : MvPolynomial τ R) :
+    eval g (rename (Prod.mk i) p) = eval (fun j => g (i, j)) p :=
+  eval₂_rename_prod_mk (RingHom.id _) _ _ _
+#align mv_polynomial.eval_rename_prodmk MvPolynomial.eval_rename_prod_mk
+
+end
+
+/-- Every polynomial is a polynomial in finitely many variables. -/
+theorem exists_finset_rename (p : MvPolynomial σ R) :
+    ∃ (s : Finset σ)(q : MvPolynomial { x // x ∈ s } R), p = rename (↑) q := by
+  apply induction_on p
+  · intro r
+    exact ⟨∅, C r, by rw [rename_C]⟩
+  · rintro p q ⟨s, p, rfl⟩ ⟨t, q, rfl⟩
+    refine' ⟨s ∪ t, ⟨_, _⟩⟩
+    ·
+      refine' rename (Subtype.map id _) p + rename (Subtype.map id _) q <;>
+        simp (config := { contextual := true }) only [id.def, true_or_iff, or_true_iff,
+          Finset.mem_union, forall_true_iff]
+    · simp only [rename_rename, AlgHom.map_add]
+      rfl
+  · rintro p n ⟨s, p, rfl⟩
+    refine' ⟨insert n s, ⟨_, _⟩⟩
+    · refine' rename (Subtype.map id _) p * X ⟨n, s.mem_insert_self n⟩
+      simp (config := { contextual := true }) only [id.def, or_true_iff, Finset.mem_insert,
+        forall_true_iff]
+    · simp only [rename_rename, rename_X, Subtype.coe_mk, AlgHom.map_mul]
+      rfl
+#align mv_polynomial.exists_finset_rename MvPolynomial.exists_finset_rename
+
+/-- `exists_finset_rename` for two polyonomials at once: for any two polynomials `p₁`, `p₂` in a
+  polynomial semiring `R[σ]` of possibly infinitely many variables, `exists_finset_rename₂` yields
+  a finite subset `s` of `σ` such that both `p₁` and `p₂` are contained in the polynomial semiring
+  `R[s]` of finitely many variables. -/
+theorem exists_finset_rename₂ (p₁ p₂ : MvPolynomial σ R) :
+    ∃ (s : Finset σ)(q₁ q₂ : MvPolynomial s R), p₁ = rename (↑) q₁ ∧ p₂ = rename (↑) q₂ := by
+  obtain ⟨s₁, q₁, rfl⟩ := exists_finset_rename p₁
+  obtain ⟨s₂, q₂, rfl⟩ := exists_finset_rename p₂
+  classical
+    use s₁ ∪ s₂
+    use rename (Set.inclusion <| s₁.subset_union_left s₂) q₁
+    use rename (Set.inclusion <| s₁.subset_union_right s₂) q₂
+    constructor -- porting note: was `<;> simp <;> rfl` but Lean couldn't infer the arguments
+    · rw [rename_rename (Set.inclusion <| s₁.subset_union_left s₂)]
+      rfl
+    · rw [rename_rename (Set.inclusion <| s₁.subset_union_right s₂)]
+      rfl
+#align mv_polynomial.exists_finset_rename₂ MvPolynomial.exists_finset_rename₂
+
+/-- Every polynomial is a polynomial in finitely many variables. -/
+theorem exists_fin_rename (p : MvPolynomial σ R) :
+    ∃ (n : ℕ)(f : Fin n → σ) (_hf : Injective f)(q : MvPolynomial (Fin n) R), p = rename f q := by
+  obtain ⟨s, q, rfl⟩ := exists_finset_rename p
+  let n := Fintype.card { x // x ∈ s }
+  let e := Fintype.equivFin { x // x ∈ s }
+  refine' ⟨n, (↑) ∘ e.symm, Subtype.val_injective.comp e.symm.injective, rename e q, _⟩
+  rw [← rename_rename, rename_rename e]
+  simp only [Function.comp, Equiv.symm_apply_apply, rename_rename]
+#align mv_polynomial.exists_fin_rename MvPolynomial.exists_fin_rename
+
+end Rename
+
+theorem eval₂_cast_comp (f : σ → τ) (c : ℤ →+* R) (g : τ → R) (p : MvPolynomial σ ℤ) :
+    eval₂ c (g ∘ f) p = eval₂ c g (rename f p) := by
+  apply MvPolynomial.induction_on p (fun n => by simp only [eval₂_C, rename_C])
+    (fun p q hp hq => by simp only [hp, hq, rename, eval₂_add, AlgHom.map_add])
+    fun p n hp => by simp only [eval₂_mul, hp, eval₂_X, comp_apply, map_mul, rename_X, eval₂_mul]
+#align mv_polynomial.eval₂_cast_comp MvPolynomial.eval₂_cast_comp
+
+section Coeff
+
+@[simp]
+theorem coeff_rename_mapDomain (f : σ → τ) (hf : Injective f) (φ : MvPolynomial σ R) (d : σ →₀ ℕ) :
+    (rename f φ).coeff (d.mapDomain f) = φ.coeff d := by
+  apply φ.induction_on' (P := fun ψ => coeff (Finsupp.mapDomain f d) ((rename f) ψ) = coeff d ψ)
+  -- Lean could no longer infer the motive
+  · intro u r
+    rw [rename_monomial, coeff_monomial, coeff_monomial]
+    simp only [(Finsupp.mapDomain_injective hf).eq_iff]
+  · intros
+    simp only [*, AlgHom.map_add, coeff_add]
+#align mv_polynomial.coeff_rename_map_domain MvPolynomial.coeff_rename_mapDomain
+
+theorem coeff_rename_eq_zero (f : σ → τ) (φ : MvPolynomial σ R) (d : τ →₀ ℕ)
+    (h : ∀ u : σ →₀ ℕ, u.mapDomain f = d → φ.coeff u = 0) : (rename f φ).coeff d = 0 := by
+  rw [rename_eq, ← not_mem_support_iff]
+  intro H
+  replace H := mapDomain_support H
+  rw [Finset.mem_image] at H
+  obtain ⟨u, hu, rfl⟩ := H
+  specialize h u rfl
+  simp at h hu
+  contradiction
+#align mv_polynomial.coeff_rename_eq_zero MvPolynomial.coeff_rename_eq_zero
+
+theorem coeff_rename_ne_zero (f : σ → τ) (φ : MvPolynomial σ R) (d : τ →₀ ℕ)
+    (h : (rename f φ).coeff d ≠ 0) : ∃ u : σ →₀ ℕ, u.mapDomain f = d ∧ φ.coeff u ≠ 0 := by
+  contrapose! h
+  apply coeff_rename_eq_zero _ _ _ h
+#align mv_polynomial.coeff_rename_ne_zero MvPolynomial.coeff_rename_ne_zero
+
+@[simp]
+theorem constantCoeff_rename {τ : Type _} (f : σ → τ) (φ : MvPolynomial σ R) :
+    constantCoeff (rename f φ) = constantCoeff φ := by
+  apply φ.induction_on
+  · intro a
+    simp only [constantCoeff_C, rename_C]
+  · intro p q hp hq
+    simp only [hp, hq, RingHom.map_add, AlgHom.map_add]
+  · intro p n hp
+    simp only [hp, rename_X, constantCoeff_X, RingHom.map_mul, AlgHom.map_mul]
+#align mv_polynomial.constant_coeff_rename MvPolynomial.constantCoeff_rename
+
+end Coeff
+
+section Support
+
+theorem support_rename_of_injective {p : MvPolynomial σ R} {f : σ → τ} (h : Function.Injective f) :
+    (rename f p).support = Finset.image (Finsupp.mapDomain f) p.support := by
+  rw [rename_eq]
+  exact Finsupp.mapDomain_support_of_injective (mapDomain_injective h) _
+#align mv_polynomial.support_rename_of_injective MvPolynomial.support_rename_of_injective
+
+end Support
+
+end MvPolynomial