Documentation

Mathlib.NumberTheory.NumberField.Embeddings

Embeddings of number fields #

This file defines the embeddings of a number field into an algebraic closed field.

Main Definitions and Results #

Tags #

number field, embeddings, places, infinite places

There are finitely many embeddings of a number field.

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The number of embeddings of a number field is equal to its finrank.

theorem NumberField.Embeddings.range_eval_eq_rootSet_minpoly (K : Type u_1) (A : Type u_2) [Field K] [NumberField K] [Field A] [Algebra A] [IsAlgClosed A] (x : K) :
(Set.range fun (φ : K →+* A) => φ x) = Polynomial.rootSet (minpoly x) A

Let A be an algebraically closed field and let x ∈ K, with K a number field. The images of x by the embeddings of K in A are exactly the roots in A of the minimal polynomial of x over .

theorem NumberField.Embeddings.finite_of_norm_le (K : Type u_1) [Field K] [NumberField K] (A : Type u_2) [NormedField A] [IsAlgClosed A] [NormedAlgebra A] (B : ) :
Set.Finite {x : K | IsIntegral x ∀ (φ : K →+* A), φ x B}

Let B be a real number. The set of algebraic integers in K whose conjugates are all smaller in norm than B is finite.

theorem NumberField.Embeddings.pow_eq_one_of_norm_eq_one (K : Type u_1) [Field K] [NumberField K] (A : Type u_2) [NormedField A] [IsAlgClosed A] [NormedAlgebra A] {x : K} (hxi : IsIntegral x) (hx : ∀ (φ : K →+* A), φ x = 1) :
∃ (n : ) (_ : 0 < n), x ^ n = 1

An algebraic integer whose conjugates are all of norm one is a root of unity.

def NumberField.place {K : Type u_1} [Field K] {A : Type u_2} [NormedDivisionRing A] [Nontrivial A] (φ : K →+* A) :

An embedding into a normed division ring defines a place of K

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    theorem NumberField.place_apply {K : Type u_1} [Field K] {A : Type u_2} [NormedDivisionRing A] [Nontrivial A] (φ : K →+* A) (x : K) :
    @[reducible]

    The conjugate of a complex embedding as a complex embedding.

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      @[reducible]

      An embedding into is real if it is fixed by complex conjugation.

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        A real embedding as a ring homomorphism from K to .

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          theorem NumberField.ComplexEmbedding.exists_comp_symm_eq_of_comp_eq {K : Type u_1} [Field K] {k : Type u_2} [Field k] [Algebra k K] [IsGalois k K] (φ : K →+* ) (ψ : K →+* ) (h : RingHom.comp φ (algebraMap k K) = RingHom.comp ψ (algebraMap k K)) :
          ∃ (σ : K ≃ₐ[k] K), RingHom.comp φ (AlgEquiv.symm σ) = ψ
          def NumberField.ComplexEmbedding.IsConj {K : Type u_1} [Field K] {k : Type u_2} [Field k] [Algebra k K] (φ : K →+* ) (σ : K ≃ₐ[k] K) :

          IsConj φ σ states that σ : K ≃ₐ[k] K is the conjugation under the embedding φ : K →+* ℂ.

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            theorem NumberField.ComplexEmbedding.IsConj.eq {K : Type u_1} [Field K] {k : Type u_2} [Field k] [Algebra k K] {φ : K →+* } {σ : K ≃ₐ[k] K} (h : NumberField.ComplexEmbedding.IsConj φ σ) (x : K) :
            φ (σ x) = star (φ x)
            theorem NumberField.ComplexEmbedding.IsConj.ext {K : Type u_1} [Field K] {k : Type u_2} [Field k] [Algebra k K] {φ : K →+* } {σ₁ : K ≃ₐ[k] K} {σ₂ : K ≃ₐ[k] K} (h₁ : NumberField.ComplexEmbedding.IsConj φ σ₁) (h₂ : NumberField.ComplexEmbedding.IsConj φ σ₂) :
            σ₁ = σ₂
            theorem NumberField.ComplexEmbedding.IsConj.ext_iff {K : Type u_1} [Field K] {k : Type u_2} [Field k] [Algebra k K] {φ : K →+* } {σ₁ : K ≃ₐ[k] K} {σ₂ : K ≃ₐ[k] K} (h₁ : NumberField.ComplexEmbedding.IsConj φ σ₁) :
            def NumberField.InfinitePlace (K : Type u_2) [Field K] :
            Type u_2

            An infinite place of a number field K is a place associated to a complex embedding.

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              noncomputable def NumberField.InfinitePlace.mk {K : Type u_2} [Field K] (φ : K →+* ) :

              Return the infinite place defined by a complex embedding φ.

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                @[simp]
                theorem NumberField.InfinitePlace.apply {K : Type u_2} [Field K] (φ : K →+* ) (x : K) :

                For an infinite place w, return an embedding φ such that w = infinite_place φ .

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                  theorem NumberField.InfinitePlace.eq_iff_eq {K : Type u_2} [Field K] (x : K) (r : ) :
                  (∀ (w : NumberField.InfinitePlace K), w x = r) ∀ (φ : K →+* ), φ x = r
                  theorem NumberField.InfinitePlace.le_iff_le {K : Type u_2} [Field K] (x : K) (r : ) :
                  (∀ (w : NumberField.InfinitePlace K), w x r) ∀ (φ : K →+* ), φ x r
                  theorem NumberField.InfinitePlace.pos_iff {K : Type u_2} [Field K] {w : NumberField.InfinitePlace K} {x : K} :
                  0 < w x x 0

                  An infinite place is real if it is defined by a real embedding.

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                    An infinite place is complex if it is defined by a complex (ie. not real) embedding.

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                      noncomputable def NumberField.InfinitePlace.mult {K : Type u_2} [Field K] (w : NumberField.InfinitePlace K) :

                      The multiplicity of an infinite place, that is the number of distinct complex embeddings that define it, see card_filter_mk_eq.

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                        The map from real embeddings to real infinite places as an equiv

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                          The map from nonreal embeddings to complex infinite places

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                            @[simp]
                            theorem NumberField.InfinitePlace.mkReal_coe {K : Type u_2} [Field K] (φ : { φ : K →+* // NumberField.ComplexEmbedding.IsReal φ }) :
                            (NumberField.InfinitePlace.mkReal φ) = NumberField.InfinitePlace.mk φ

                            The infinite part of the product formula : for x ∈ K, we have Π_w ‖x‖_w = |norm(x)| where ‖·‖_w is the normalized absolute value for w.

                            @[inline, reducible]
                            noncomputable abbrev NumberField.InfinitePlace.NrRealPlaces (K : Type u_2) [Field K] [NumberField K] :

                            The number of infinite real places of the number field K.

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                              noncomputable abbrev NumberField.InfinitePlace.NrComplexPlaces (K : Type u_2) [Field K] [NumberField K] :

                              The number of infinite complex places of the number field K.

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                                The restriction of an infinite place along an embedding.

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                                  The action of the galois group on infinite places.

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                                  theorem NumberField.InfinitePlace.smul_apply {k : Type u_1} [Field k] {K : Type u_2} [Field K] [Algebra k K] (σ : K ≃ₐ[k] K) (w : NumberField.InfinitePlace K) (x : K) :
                                  (σ w) x = w ((AlgEquiv.symm σ) x)
                                  theorem NumberField.InfinitePlace.ComplexEmbedding.exists_comp_symm_eq_of_comp_eq {k : Type u_1} [Field k] {K : Type u_2} [Field K] [Algebra k K] [IsGalois k K] (φ : K →+* ) (ψ : K →+* ) (h : RingHom.comp φ (algebraMap k K) = RingHom.comp ψ (algebraMap k K)) :
                                  ∃ (σ : K ≃ₐ[k] K), RingHom.comp φ (AlgEquiv.symm σ) = ψ

                                  The orbits of infinite places under the action of the galois group are indexed by the infinite places of the base field.

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                                    theorem NumberField.InfinitePlace.orbitRelEquiv_apply_mk'' {k : Type u_1} [Field k] {K : Type u_2} [Field K] [Algebra k K] [IsGalois k K] (w : NumberField.InfinitePlace K) :
                                    NumberField.InfinitePlace.orbitRelEquiv (Quotient.mk'' w) = NumberField.InfinitePlace.comap w (algebraMap k K)

                                    An infinite place is unramified in a field extension if the restriction has the same multiplicity.

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                                      A infinite place of the base field is unramified in a field extension if every infinite place over it is unramified.

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                                        class IsUnramifiedAtInfinitePlaces (k : Type u_1) [Field k] (K : Type u_2) [Field K] [Algebra k K] :

                                        A field extension is unramified at infinite places if every infinite place is unramified.

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                                          theorem IsUnramifiedAtInfinitePlaces.bot (k : Type u_1) [Field k] (K : Type u_2) [Field K] (F : Type u_3) [Field F] [Algebra k K] [Algebra k F] [Algebra K F] [IsScalarTower k K F] [h₁ : IsUnramifiedAtInfinitePlaces k F] (h : Algebra.IsAlgebraic K F) :