feat: Refactor and add some easy lemmas about Galois theory

Mathlib/FieldTheory/Galois/Basic.lean

@@ -184,11 +184,15 @@ namespace IntermediateField
 def fixedField : IntermediateField F E :=
   FixedPoints.intermediateField H
 
-theorem mem_fixedField_iff (x) :
+@[simp] lemma mem_fixedField_iff (x) :
     x ∈ fixedField H ↔ ∀ f ∈ H, f x = x := by
   show x ∈ MulAction.fixedPoints H E ↔ _
   simp only [MulAction.mem_fixedPoints, Subtype.forall, Subgroup.mk_smul, AlgEquiv.smul_def]
 
+@[simp] lemma fixedField_bot : fixedField (⊥ : Subgroup (E ≃ₐ[F] E)) = ⊤ := by
+  ext
+  simp
+
 theorem finrank_fixedField_eq_card [FiniteDimensional F E] [DecidablePred (· ∈ H)] :
     finrank (fixedField H) E = Fintype.card H :=
   FixedPoints.finrank_eq_card H E
@@ -205,6 +209,17 @@ lemma fixingSubgroup_anti : Antitone (IntermediateField.fixingSubgroup (F := F)
   rw [← le_iff_le]
   exact le_trans h ((le_iff_le _ _).mpr (le_refl K'.fixingSubgroup))
 
+@[simp] lemma mem_fixingSubgroup_iff (σ) : σ ∈ fixingSubgroup K ↔ ∀ x ∈ K, σ x = x :=
+  _root_.mem_fixingSubgroup_iff _
+
+@[simp] lemma fixingSubgroup_top : fixingSubgroup (⊤ : IntermediateField F E) = ⊥ := by
+  ext
+  simp [DFunLike.ext_iff]
+
+@[simp] lemma fixingSubgroup_bot : fixingSubgroup (⊥ : IntermediateField F E) = ⊤ := by
+  ext
+  simp [mem_bot]
+
 /-- The fixing subgroup of `K : IntermediateField F E` is isomorphic to `E ≃ₐ[K] E` -/
 def fixingSubgroupEquiv : fixingSubgroup K ≃* E ≃ₐ[K] E where
   toFun ϕ := { AlgEquiv.toRingEquiv (ϕ : E ≃ₐ[F] E) with commutes' := ϕ.mem }
@@ -258,6 +273,19 @@ theorem fixedField_fixingSubgroup [FiniteDimensional F E] [h : IsGalois F E] :
     Fintype.card_congr (IntermediateField.fixingSubgroupEquiv K).toEquiv]
   exact (card_aut_eq_finrank K E).symm
 
+@[simp] lemma fixedField_top [IsGalois F E] [FiniteDimensional F E] :
+    fixedField (⊤ : Subgroup (E ≃ₐ[F] E)) = ⊥ := by
+  rw [← fixingSubgroup_bot, fixedField_fixingSubgroup]
+
+theorem mem_bot_iff_fixed [IsGalois F E] [FiniteDimensional F E] (x : E) :
+    x ∈ (⊥ : IntermediateField F E) ↔ ∀ f : E ≃ₐ[F] E, f x = x := by
+  rw [← fixedField_top, mem_fixedField_iff]
+  simp only [Subgroup.mem_top, forall_const]
+
+theorem mem_range_algebraMap_iff_fixed [IsGalois F E] [FiniteDimensional F E] (x : E) :
+    x ∈ Set.range (algebraMap F E) ↔ ∀ f : E ≃ₐ[F] E, f x = x :=
+  mem_bot_iff_fixed x
+
 theorem card_fixingSubgroup_eq_finrank [DecidablePred (· ∈ IntermediateField.fixingSubgroup K)]
     [FiniteDimensional F E] [IsGalois F E] :
     Fintype.card (IntermediateField.fixingSubgroup K) = finrank K E := by
@@ -309,8 +337,8 @@ open scoped Pointwise
 
 lemma IntermediateField.restrictNormalHom_ker (E : IntermediateField K L) [Normal K E] :
     (restrictNormalHom E).ker = E.fixingSubgroup := by
-  simp [fixingSubgroup, Subgroup.ext_iff, AlgEquiv.ext_iff, Subtype.ext_iff,
-    restrictNormalHom_apply, mem_fixingSubgroup_iff]
+  simp [Subgroup.ext_iff, AlgEquiv.ext_iff, Subtype.ext_iff,
+    restrictNormalHom_apply]
 
 namespace IsGalois
 

Mathlib/FieldTheory/KrullTopology.lean

@@ -83,6 +83,7 @@ theorem IntermediateField.finiteDimensional_bot (K L : Type*) [Field K] [Field L
   .of_rank_eq_one IntermediateField.rank_bot
 
 /-- This lemma says that `Gal(L/K) = L ≃ₐ[K] L` -/
+@[deprecated IntermediateField.fixingSubgroup_bot (since := "2025-03-12")]
 theorem IntermediateField.fixingSubgroup.bot {K L : Type*} [Field K] [Field L] [Algebra K L] :
     IntermediateField.fixingSubgroup (⊥ : IntermediateField K L) = ⊤ := by
   ext f
@@ -95,7 +96,7 @@ theorem IntermediateField.fixingSubgroup.bot {K L : Type*} [Field K] [Field L] [
 /-- If `L/K` is a field extension, then we have `Gal(L/K) ∈ fixedByFinite K L` -/
 theorem top_fixedByFinite {K L : Type*} [Field K] [Field L] [Algebra K L] :
     ⊤ ∈ fixedByFinite K L :=
-  ⟨⊥, IntermediateField.finiteDimensional_bot K L, IntermediateField.fixingSubgroup.bot⟩
+  ⟨⊥, IntermediateField.finiteDimensional_bot K L, IntermediateField.fixingSubgroup_bot⟩
 
 /-- If `E1` and `E2` are finite-dimensional intermediate fields, then so is their compositum.
 This rephrases a result already in mathlib so that it is compatible with our type classes -/
@@ -104,11 +105,6 @@ theorem finiteDimensional_sup {K L : Type*} [Field K] [Field L] [Algebra K L]
     FiniteDimensional K (↥(E1 ⊔ E2)) :=
   IntermediateField.finiteDimensional_sup E1 E2
 
-/-- An element of `L ≃ₐ[K] L` is in `Gal(L/E)` if and only if it fixes every element of `E`. -/
-theorem IntermediateField.mem_fixingSubgroup_iff {K L : Type*} [Field K] [Field L] [Algebra K L]
-    (E : IntermediateField K L) (σ : L ≃ₐ[K] L) : σ ∈ E.fixingSubgroup ↔ ∀ x : L, x ∈ E → σ x = x :=
-  ⟨fun hσ x hx => hσ ⟨x, hx⟩, fun h ⟨x, hx⟩ => h x hx⟩
-
 /-- The map `E ↦ Gal(L/E)` is inclusion-reversing -/
 theorem IntermediateField.fixingSubgroup.antimono {K L : Type*} [Field K] [Field L] [Algebra K L]
     {E1 E2 : IntermediateField K L} (h12 : E1 ≤ E2) : E2.fixingSubgroup ≤ E1.fixingSubgroup := by
@@ -283,15 +279,6 @@ instance {K L : Type*} [Field K] [Field L] [Algebra K L] [Algebra.IsIntegral K L
 
 end TotallyDisconnected
 
-@[simp] lemma IntermediateField.fixingSubgroup_top (K L : Type*) [Field K] [Field L] [Algebra K L] :
-    IntermediateField.fixingSubgroup (⊤ : IntermediateField K L) = ⊥ := by
-  ext
-  simp [mem_fixingSubgroup_iff, DFunLike.ext_iff]
-
-@[simp] lemma IntermediateField.fixingSubgroup_bot (K L : Type*) [Field K] [Field L] [Algebra K L] :
-    IntermediateField.fixingSubgroup (⊥ : IntermediateField K L) = ⊤ := by
-  ext
-  simp [mem_fixingSubgroup_iff, mem_bot]
 
 instance krullTopology_discreteTopology_of_finiteDimensional (K L : Type*) [Field K] [Field L]
     [Algebra K L] [FiniteDimensional K L] : DiscreteTopology (L ≃ₐ[K] L) := by