Algebraically Closed Field

In this file we define the typeclass for algebraically closed fields and algebraic closures, and prove some of their properties.

Main Definitions

• IsAlgClosed k is the typeclass saying k is an algebraically closed field, i.e. every polynomial in k splits.

• IsAlgClosure R K is the typeclass saying K is an algebraic closure of R, where R is a commutative ring. This means that the map from R to K is injective, and K is algebraically closed and algebraic over R

• IsAlgClosed.lift is a map from an algebraic extension L of R, into any algebraically closed extension of R.

• IsAlgClosure.equiv is a proof that any two algebraic closures of the same field are isomorphic.

Tags

algebraic closure, algebraically closed

TODO

• Prove that if K / k is algebraic, and any monic irreducible polynomial over k has a root in K, then K is algebraically closed (in fact an algebraic closure of k).

  Reference: , Theorem 2

class IsAlgClosed (k : Type u) [Field k] : Prop

Typeclass for algebraically closed fields.

To show Polynomial.Splits p f for an arbitrary ring homomorphism f, see IsAlgClosed.splits_codomain and IsAlgClosed.splits_domain.

• splits : ∀ (p : Polynomial k), Polynomial.Splits (RingHom.id k) p

theorem IsAlgClosed.splits {k : Type u} [Field k] [self : IsAlgClosed k] (p : Polynomial k) : Polynomial.Splits (RingHom.id k) p

theorem IsAlgClosed.splits_codomain {k : Type u_1} {K : Type u_2} [Field k] [IsAlgClosed k] [Field K] {f : K →+* k} (p : Polynomial K) : Polynomial.Splits f p

Every polynomial splits in the field extension f : K →+* k if k is algebraically closed.

See also IsAlgClosed.splits_domain for the case where K is algebraically closed.

theorem IsAlgClosed.splits_domain {k : Type u_1} {K : Type u_2} [Field k] [IsAlgClosed k] [Field K] {f : k →+* K} (p : Polynomial k) : Polynomial.Splits f p

Every polynomial splits in the field extension f : K →+* k if K is algebraically closed.

See also IsAlgClosed.splits_codomain for the case where k is algebraically closed.

theorem IsAlgClosed.exists_root {k : Type u} [Field k] [IsAlgClosed k] (p : Polynomial k) (hp : p.degree ≠ 0) : ∃ (x : k), p.IsRoot x

theorem IsAlgClosed.exists_pow_nat_eq {k : Type u} [Field k] [IsAlgClosed k] (x : k) {n : ℕ} (hn : 0 < n) : ∃ (z : k), z ^ n = x

theorem IsAlgClosed.exists_eq_mul_self {k : Type u} [Field k] [IsAlgClosed k] (x : k) : ∃ (z : k), x = z * z

theorem IsAlgClosed.roots_eq_zero_iff {k : Type u} [Field k] [IsAlgClosed k] {p : Polynomial k} : p.roots = 0 ↔ p = Polynomial.C (p.coeff 0)

theorem IsAlgClosed.exists_eval₂_eq_zero_of_injective {k : Type u} [Field k] {R : Type u_1} [Ring R] [IsAlgClosed k] (f : R →+* k) (hf : Function.Injective ⇑f) (p : Polynomial R) (hp : p.degree ≠ 0) : ∃ (x : k), Polynomial.eval₂ f x p = 0

theorem IsAlgClosed.exists_eval₂_eq_zero {k : Type u} [Field k] {R : Type u_1} [Field R] [IsAlgClosed k] (f : R →+* k) (p : Polynomial R) (hp : p.degree ≠ 0) : ∃ (x : k), Polynomial.eval₂ f x p = 0

theorem IsAlgClosed.exists_aeval_eq_zero_of_injective (k : Type u) [Field k] {R : Type u_1} [CommRing R] [IsAlgClosed k] [Algebra R k] (hinj : Function.Injective ⇑(algebraMap R k)) (p : Polynomial R) (hp : p.degree ≠ 0) : ∃ (x : k), (Polynomial.aeval x) p = 0

theorem IsAlgClosed.exists_aeval_eq_zero (k : Type u) [Field k] {R : Type u_1} [Field R] [IsAlgClosed k] [Algebra R k] (p : Polynomial R) (hp : p.degree ≠ 0) : ∃ (x : k), (Polynomial.aeval x) p = 0

theorem IsAlgClosed.of_exists_root (k : Type u) [Field k] (H : ∀ (p : Polynomial k), p.Monic → Irreducible p → ∃ (x : k), Polynomial.eval x p = 0) : IsAlgClosed k

theorem IsAlgClosed.of_ringEquiv (k : Type u) [Field k] (k' : Type u) [Field k'] (e : k ≃+* k') [IsAlgClosed k] : IsAlgClosed k'

theorem IsAlgClosed.degree_eq_one_of_irreducible (k : Type u) [Field k] [IsAlgClosed k] {p : Polynomial k} (hp : Irreducible p) : p.degree = 1

theorem IsAlgClosed.algebraMap_surjective_of_isIntegral {k : Type u_1} {K : Type u_2} [Field k] [Ring K] [IsDomain K] [hk : IsAlgClosed k] [Algebra k K] (hf : Algebra.IsIntegral k K) : Function.Surjective ⇑(algebraMap k K)

theorem IsAlgClosed.algebraMap_surjective_of_isIntegral' {k : Type u_1} {K : Type u_2} [Field k] [CommRing K] [IsDomain K] [IsAlgClosed k] (f : k →+* K) (hf : f.IsIntegral) : Function.Surjective ⇑f

theorem IsAlgClosed.algebraMap_surjective_of_isAlgebraic {k : Type u_1} {K : Type u_2} [Field k] [Ring K] [IsDomain K] [IsAlgClosed k] [Algebra k K] (hf : Algebra.IsAlgebraic k K) : Function.Surjective ⇑(algebraMap k K)

theorem IntermediateField.eq_bot_of_isAlgClosed_of_isAlgebraic {k : Type u_1} {K : Type u_2} [Field k] [Field K] [IsAlgClosed k] [Algebra k K] (L : IntermediateField k K) (hf : Algebra.IsAlgebraic k ↥L) : L = ⊥

If k is algebraically closed, K / k is a field extension, L / k is an intermediate field which is algebraic, then L is equal to k. A corollary of IsAlgClosed.algebraMap_surjective_of_isAlgebraic.

class IsAlgClosure (R : Type u) (K : Type v) [CommRing R] [Field K] [Algebra R K] [NoZeroSMulDivisors R K] : Prop

Typeclass for an extension being an algebraic closure.

• alg_closed : IsAlgClosed K
• algebraic : Algebra.IsAlgebraic R K

theorem IsAlgClosure.alg_closed (R : Type u) {K : Type v} [CommRing R] [Field K] [Algebra R K] [NoZeroSMulDivisors R K] [self : IsAlgClosure R K] : IsAlgClosed K

theorem IsAlgClosure.algebraic {R : Type u} {K : Type v} [CommRing R] [Field K] [Algebra R K] [NoZeroSMulDivisors R K] [self : IsAlgClosure R K] : Algebra.IsAlgebraic R K

theorem isAlgClosure_iff (k : Type u) [Field k] (K : Type v) [Field K] [Algebra k K] : IsAlgClosure k K ↔ IsAlgClosed K ∧ Algebra.IsAlgebraic k K

instance IsAlgClosure.normal (R : Type u_1) (K : Type u_2) [Field R] [Field K] [Algebra R K] [IsAlgClosure R K] : Normal R K

Equations

• ⋯ = ⋯

instance IsAlgClosure.separable (R : Type u_1) (K : Type u_2) [Field R] [Field K] [Algebra R K] [IsAlgClosure R K] [CharZero R] : IsSeparable R K

Equations

• ⋯ = ⋯

theorem IsAlgClosed.surjective_comp_algebraMap_of_isAlgebraic {K : Type u} [Field K] {L : Type v} {M : Type w} [Field L] [Algebra K L] [Field M] [Algebra K M] [IsAlgClosed M] {E : Type u_1} [Field E] [Algebra K E] [Algebra L E] [IsScalarTower K L E] (hE : Algebra.IsAlgebraic L E) : Function.Surjective fun (φ : E →ₐ[K] M) => φ.comp (IsScalarTower.toAlgHom K L E)

If E/L/K is a tower of field extensions with E/L algebraic, and if M is an algebraically closed extension of K, then any embedding of L/K into M/K extends to an embedding of E/K. Known as the extension lemma in https://math.stackexchange.com/a/687914.

@[irreducible]

noncomputable def IsAlgClosed.lift {M : Type u_1} [Field M] [IsAlgClosed M] {R : Type u_2} [CommRing R] {S : Type u_3} [CommRing S] [IsDomain S] [Algebra R S] [Algebra R M] [NoZeroSMulDivisors R S] [NoZeroSMulDivisors R M] (hS : Algebra.IsAlgebraic R S) : S →ₐ[R] M

A (random) homomorphism from an algebraic extension of R into an algebraically closed extension of R.

Equations

• @IsAlgClosed.lift = IsAlgClosed.wrapped✝.1

theorem IsAlgClosed.lift_def {M : Type u_1} [Field M] [IsAlgClosed M] {R : Type u_2} [CommRing R] {S : Type u_3} [CommRing S] [IsDomain S] [Algebra R S] [Algebra R M] [NoZeroSMulDivisors R S] [NoZeroSMulDivisors R M] (hS : Algebra.IsAlgebraic R S) : IsAlgClosed.lift hS = let_fun this := ⋯; let_fun this_1 := ⋯; let_fun this_2 := ⋯; let f := IsAlgClosed.lift_aux (FractionRing R) (FractionRing S) M this_2; (AlgHom.restrictScalars R f).comp (AlgHom.restrictScalars R (Algebra.ofId S (FractionRing S)))

noncomputable instance IsAlgClosed.perfectRing (k : Type u) [Field k] (p : ℕ) [Fact p.Prime] [CharP k p] [IsAlgClosed k] : PerfectRing k p

Equations

• ⋯ = ⋯

noncomputable instance IsAlgClosed.perfectField (k : Type u) [Field k] [IsAlgClosed k] : PerfectField k

Equations

• ⋯ = ⋯

instance IsAlgClosed.instInfinite {K : Type u_1} [Field K] [IsAlgClosed K] : Infinite K

Algebraically closed fields are infinite since Xⁿ⁺¹ - 1 is separable when #K = n

Equations

• ⋯ = ⋯

noncomputable def IsAlgClosure.equiv (R : Type u) [CommRing R] (L : Type v) (M : Type w) [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra R L] [NoZeroSMulDivisors R L] [IsAlgClosure R L] : L ≃ₐ[R] M

A (random) isomorphism between two algebraic closures of R.

Equations

• IsAlgClosure.equiv R L M = AlgEquiv.ofBijective (IsAlgClosed.lift ⋯) ⋯

theorem IsAlgClosure.ofAlgebraic (K : Type u_1) (J : Type u_2) (L : Type v) [Field K] [Field J] [Field L] [Algebra K J] [Algebra J L] [IsAlgClosure J L] [Algebra K L] [IsScalarTower K J L] (hKJ : Algebra.IsAlgebraic K J) : IsAlgClosure K L

If J is an algebraic extension of K and L is an algebraic closure of J, then it is also an algebraic closure of K.

noncomputable def IsAlgClosure.equivOfAlgebraic' (R : Type u) (S : Type u_3) (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] [Algebra R S] [Algebra R L] [IsScalarTower R S L] [Nontrivial S] [NoZeroSMulDivisors R S] (hRL : Algebra.IsAlgebraic R L) : L ≃ₐ[R] M

A (random) isomorphism between an algebraic closure of R and an algebraic closure of an algebraic extension of R

Equations

• IsAlgClosure.equivOfAlgebraic' R S L M hRL = IsAlgClosure.equiv R L M

noncomputable def IsAlgClosure.equivOfAlgebraic (K : Type u_1) (J : Type u_2) (L : Type v) (M : Type w) [Field K] [Field J] [Field L] [Field M] [Algebra K M] [IsAlgClosure K M] [Algebra K J] [Algebra J L] [IsAlgClosure J L] [Algebra K L] [IsScalarTower K J L] (hKJ : Algebra.IsAlgebraic K J) : L ≃ₐ[K] M

A (random) isomorphism between an algebraic closure of K and an algebraic closure of an algebraic extension of K

Equations

• IsAlgClosure.equivOfAlgebraic K J L M hKJ = IsAlgClosure.equivOfAlgebraic' K J L M ⋯

noncomputable def IsAlgClosure.equivOfEquivAux {R : Type u} {S : Type u_3} (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] (hSR : S ≃+* R) : { e : L ≃+* M // e.toRingHom.comp (algebraMap S L) = (algebraMap R M).comp hSR.toRingHom }

Used in the definition of equivOfEquiv

Equations

• IsAlgClosure.equivOfEquivAux L M hSR = ⟨↑(IsAlgClosure.equivOfAlgebraic' R S L M ⋯), ⋯⟩

noncomputable def IsAlgClosure.equivOfEquiv {R : Type u} {S : Type u_3} (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] (hSR : S ≃+* R) : L ≃+* M

Algebraic closure of isomorphic fields are isomorphic

Equations

• IsAlgClosure.equivOfEquiv L M hSR = ↑(IsAlgClosure.equivOfEquivAux L M hSR)

@[simp]

theorem IsAlgClosure.equivOfEquiv_comp_algebraMap {R : Type u} {S : Type u_3} (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] (hSR : S ≃+* R) : (↑(IsAlgClosure.equivOfEquiv L M hSR)).comp (algebraMap S L) = (algebraMap R M).comp ↑hSR

@[simp]

theorem IsAlgClosure.equivOfEquiv_algebraMap {R : Type u} {S : Type u_3} (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] (hSR : S ≃+* R) (s : S) : (IsAlgClosure.equivOfEquiv L M hSR) ((algebraMap S L) s) = (algebraMap R M) (hSR s)

@[simp]

theorem IsAlgClosure.equivOfEquiv_symm_algebraMap {R : Type u} {S : Type u_3} (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] (hSR : S ≃+* R) (r : R) : (IsAlgClosure.equivOfEquiv L M hSR).symm ((algebraMap R M) r) = (algebraMap S L) (hSR.symm r)

@[simp]

theorem IsAlgClosure.equivOfEquiv_symm_comp_algebraMap {R : Type u} {S : Type u_3} (L : Type v) (M : Type w) [CommRing R] [CommRing S] [Field L] [Field M] [Algebra R M] [NoZeroSMulDivisors R M] [IsAlgClosure R M] [Algebra S L] [NoZeroSMulDivisors S L] [IsAlgClosure S L] (hSR : S ≃+* R) : (↑(IsAlgClosure.equivOfEquiv L M hSR).symm).comp (algebraMap R M) = (algebraMap S L).comp ↑hSR.symm

theorem Algebra.IsAlgebraic.range_eval_eq_rootSet_minpoly {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) [IsAlgClosed A] (x : K) : (Set.range fun (ψ : K →ₐ[F] A) => ψ x) = (minpoly F x).rootSet A

Let A be an algebraically closed field and let x ∈ K, with K/F an algebraic extension of fields. Then the images of x by the F-algebra morphisms from K to A are exactly the roots in A of the minimal polynomial of x over F.

@[simp]

theorem IntermediateField.algHomEquivAlgHomOfSplits_symm_apply {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) (L : IntermediateField F A) (hL : ∀ (x : K), Polynomial.Splits (algebraMap F ↥L) (minpoly F x)) (f : K →ₐ[F] A) : (IntermediateField.algHomEquivAlgHomOfSplits A hK L hL).symm f = f.codRestrict L.toSubalgebra ⋯

@[simp]

theorem IntermediateField.algHomEquivAlgHomOfSplits_apply {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) (L : IntermediateField F A) (hL : ∀ (x : K), Polynomial.Splits (algebraMap F ↥L) (minpoly F x)) (φ₂ : K →ₐ[F] ↥L) : (IntermediateField.algHomEquivAlgHomOfSplits A hK L hL) φ₂ = L.val.comp φ₂

def IntermediateField.algHomEquivAlgHomOfSplits {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) (L : IntermediateField F A) (hL : ∀ (x : K), Polynomial.Splits (algebraMap F ↥L) (minpoly F x)) : (K →ₐ[F] ↥L) ≃ (K →ₐ[F] A)

All F-embeddings of a field K into another field A factor through any intermediate field of A/F in which the minimal polynomial of elements of K splits.

Equations

• IntermediateField.algHomEquivAlgHomOfSplits A hK L hL = { toFun := L.val.comp, invFun := fun (f : K →ₐ[F] A) => f.codRestrict L.toSubalgebra ⋯, left_inv := ⋯, right_inv := ⋯ }

theorem IntermediateField.algHomEquivAlgHomOfSplits_apply_apply {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) (L : IntermediateField F A) (hL : ∀ (x : K), Polynomial.Splits (algebraMap F ↥L) (minpoly F x)) (f : K →ₐ[F] ↥L) (x : K) : ((IntermediateField.algHomEquivAlgHomOfSplits A hK L hL) f) x = (algebraMap (↥L) A) (f x)

noncomputable def Algebra.IsAlgebraic.algHomEquivAlgHomOfSplits {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) (L : Type u_4) [Field L] [Algebra F L] [Algebra L A] [IsScalarTower F L A] (hL : ∀ (x : K), Polynomial.Splits (algebraMap F L) (minpoly F x)) : (K →ₐ[F] L) ≃ (K →ₐ[F] A)

All F-embeddings of a field K into another field A factor through any subextension of A/F in which the minimal polynomial of elements of K splits.

Equations

• One or more equations did not get rendered due to their size.

theorem Algebra.IsAlgebraic.algHomEquivAlgHomOfSplits_apply_apply {F : Type u_1} {K : Type u_2} (A : Type u_3) [Field F] [Field K] [Field A] [Algebra F K] [Algebra F A] (hK : Algebra.IsAlgebraic F K) (L : Type u_4) [Field L] [Algebra F L] [Algebra L A] [IsScalarTower F L A] (hL : ∀ (x : K), Polynomial.Splits (algebraMap F L) (minpoly F x)) (f : K →ₐ[F] L) (x : K) : ((Algebra.IsAlgebraic.algHomEquivAlgHomOfSplits A hK L hL) f) x = (algebraMap L A) (f x)