Mathlib.FieldTheory.Finite.Polynomial

Polynomials over finite fields #

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theorem MvPolynomial.C_dvd_iff_zmod {σ : Type u_1} (n : ℕ) (φ : MvPolynomial σ ℤ) :

C ↑n ∣ φ ↔ (map (Int.castRingHom (ZMod n))) φ = 0

A polynomial over the integers is divisible by n : ℕ if and only if it is zero over ZMod n.

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theorem MvPolynomial.frobenius_zmod {σ : Type u_1} {p : ℕ} [Fact (Nat.Prime p)] (f : MvPolynomial σ (ZMod p)) :

(frobenius (MvPolynomial σ (ZMod p)) p) f = (expand p) f

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theorem MvPolynomial.expand_zmod {σ : Type u_1} {p : ℕ} [Fact (Nat.Prime p)] (f : MvPolynomial σ (ZMod p)) :

(expand p) f = f ^ p

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def MvPolynomial.indicator {K : Type u_1} {σ : Type u_2} [Fintype K] [Fintype σ] [CommRing K] (a : σ → K) :

MvPolynomial σ K

Over a field, this is the indicator function as an MvPolynomial.

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MvPolynomial.indicator a = ∏ n : σ, (1 - (MvPolynomial.X n - MvPolynomial.C (a n)) ^ (Fintype.card K - 1))

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theorem MvPolynomial.eval_indicator_apply_eq_one {K : Type u_1} {σ : Type u_2} [Fintype K] [Fintype σ] [CommRing K] (a : σ → K) :

(eval a) (indicator a) = 1

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theorem MvPolynomial.degrees_indicator {K : Type u_1} {σ : Type u_2} [Fintype K] [Fintype σ] [CommRing K] (c : σ → K) :

(indicator c).degrees ≤ ∑ s : σ, (Fintype.card K - 1) • {s}

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theorem MvPolynomial.indicator_mem_restrictDegree {K : Type u_1} {σ : Type u_2} [Fintype K] [Fintype σ] [CommRing K] (c : σ → K) :

indicator c ∈ restrictDegree σ K (Fintype.card K - 1)

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theorem MvPolynomial.eval_indicator_apply_eq_zero {K : Type u_1} {σ : Type u_2} [Fintype K] [Fintype σ] [Field K] (a b : σ → K) (h : a ≠ b) :

(eval a) (indicator b) = 0

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def MvPolynomial.evalₗ (K : Type u_1) (σ : Type u_2) [CommSemiring K] :

MvPolynomial σ K →ₗ[K] (σ → K) → K

MvPolynomial.eval as a K-linear map.

Equations

MvPolynomial.evalₗ K σ = { toFun := fun (p : MvPolynomial σ K) (e : σ → K) => (MvPolynomial.eval e) p, map_add' := ⋯, map_smul' := ⋯ }

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@[simp]

theorem MvPolynomial.evalₗ_apply (K : Type u_1) (σ : Type u_2) [CommSemiring K] (p : MvPolynomial σ K) (e : σ → K) :

(evalₗ K σ) p e = (eval e) p

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theorem MvPolynomial.map_restrict_dom_evalₗ (K : Type u_1) (σ : Type u_2) [Field K] [Fintype K] [Finite σ] :

Submodule.map (evalₗ K σ) (restrictDegree σ K (Fintype.card K - 1)) = ⊤

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def MvPolynomial.R (σ K : Type u) [Fintype K] [CommRing K] :

Type u

The submodule of multivariate polynomials whose degree of each variable is strictly less than the cardinality of K.

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MvPolynomial.R σ K = ↥(MvPolynomial.restrictDegree σ K (Fintype.card K - 1))

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noncomputable instance MvPolynomial.instAddCommGroupR (σ K : Type u) [Fintype K] [CommRing K] :

AddCommGroup (R σ K)

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MvPolynomial.instAddCommGroupR σ K = inferInstanceAs (AddCommGroup ↥(MvPolynomial.restrictDegree σ K (Fintype.card K - 1)))

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noncomputable instance MvPolynomial.instModuleR (σ K : Type u) [Fintype K] [CommRing K] :

Module K (R σ K)

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MvPolynomial.instModuleR σ K = inferInstanceAs (Module K ↥(MvPolynomial.restrictDegree σ K (Fintype.card K - 1)))

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noncomputable instance MvPolynomial.instInhabitedR (σ K : Type u) [Fintype K] [CommRing K] :

Inhabited (R σ K)

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MvPolynomial.instInhabitedR σ K = inferInstanceAs (Inhabited ↥(MvPolynomial.restrictDegree σ K (Fintype.card K - 1)))

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noncomputable def MvPolynomial.evalᵢ (σ K : Type u) [Fintype K] [CommRing K] :

R σ K →ₗ[K] (σ → K) → K

Evaluation in the MvPolynomial.R subtype.

Equations

MvPolynomial.evalᵢ σ K = MvPolynomial.evalₗ K σ ∘ₗ (MvPolynomial.restrictDegree σ K (Fintype.card K - 1)).subtype

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noncomputable instance MvPolynomial.decidableRestrictDegree (σ : Type u) (m : ℕ) :

DecidablePred fun (x : σ →₀ ℕ) => x ∈ {n : σ →₀ ℕ | ∀ (i : σ), n i ≤ m}

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MvPolynomial.decidableRestrictDegree σ m = id inferInstance

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theorem MvPolynomial.rank_R (σ K : Type u) [Fintype K] [Field K] [Fintype σ] :

Module.rank K (R σ K) = ↑(Fintype.card (σ → K))

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instance MvPolynomial.instFiniteDimensionalROfFinite (σ K : Type u) [Fintype K] [Field K] [Finite σ] :

FiniteDimensional K (R σ K)

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theorem MvPolynomial.finrank_R (σ K : Type u) [Fintype K] [Field K] [Fintype σ] :

Module.finrank K (R σ K) = Fintype.card (σ → K)

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theorem MvPolynomial.range_evalᵢ (σ K : Type u) [Fintype K] [Field K] [Finite σ] :

LinearMap.range (evalᵢ σ K) = ⊤

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theorem MvPolynomial.ker_evalₗ (σ K : Type u) [Fintype K] [Field K] [Finite σ] :

LinearMap.ker (evalᵢ σ K) = ⊥

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theorem MvPolynomial.eq_zero_of_eval_eq_zero (σ K : Type u) [Fintype K] [Field K] [Finite σ] (p : MvPolynomial σ K) (h : ∀ (v : σ → K), (eval v) p = 0) (hp : p ∈ restrictDegree σ K (Fintype.card K - 1)) :

p = 0