✨ Transition over RegularSupport

Rubin.lean

@@ -24,6 +24,7 @@ import Rubin.Topological
 import Rubin.RigidStabilizer
 import Rubin.Period
 import Rubin.AlgebraicDisjointness
+import Rubin.RegularSupport
 
 #align_import rubin
 
@@ -63,82 +64,20 @@ where
 
 end RubinActions
 
-----------------------------------------------------------------
-section Rubin.RegularSupport.RegularSupport
-
-variable [TopologicalSpace α] [Rubin.ContinuousMulAction G α]
-
-def RegularSupport.InteriorClosure (U : Set α) :=
-  interior (closure U)
-#align interior_closure Rubin.RegularSupport.InteriorClosure
-
-theorem RegularSupport.is_open_interiorClosure (U : Set α) :
-    IsOpen (Rubin.RegularSupport.InteriorClosure U) :=
-  isOpen_interior
-#align is_open_interior_closure Rubin.RegularSupport.is_open_interiorClosure
+section proposition_2_1
 
-theorem RegularSupport.interiorClosure_mono {U V : Set α} :
-    U ⊆ V → Rubin.RegularSupport.InteriorClosure U ⊆ Rubin.RegularSupport.InteriorClosure V :=
-  interior_mono ∘ closure_mono
-#align interior_closure_mono Rubin.RegularSupport.interiorClosure_mono
+variable {G α : Type _}
+variable [Group G]
+variable [MulAction G α]
+variable [TopologicalSpace α]
+variable [T2Space α]
 
-def is_regular_open (U : Set α) :=
-  Rubin.RegularSupport.InteriorClosure U = U
-#align set.is_regular_open Rubin.is_regular_open
+lemma proposition_2_1 (f : G) :
+  Set.centralizer { g^12 | (g : G) (_ : AlgebraicallyDisjoint f g) } = RigidStabilizer G (RegularSupport α f) :=
+by
+  sorry
 
-theorem is_regular_def (U : Set α) :
-    is_regular_open U ↔ Rubin.RegularSupport.InteriorClosure U = U := by rfl
-#align set.is_regular_def Rubin.is_regular_def
-
-theorem RegularSupport.IsOpen.in_closure {U : Set α} : IsOpen U → U ⊆ interior (closure U) :=
-  by
-  intro U_open x x_in_U
-  apply interior_mono subset_closure
-  rw [U_open.interior_eq]
-  exact x_in_U
-#align is_open.in_closure Rubin.RegularSupport.IsOpen.in_closure
-
-theorem RegularSupport.IsOpen.interiorClosure_subset {U : Set α} :
-    IsOpen U → U ⊆ Rubin.RegularSupport.InteriorClosure U := fun h =>
-  (subset_interior_iff_isOpen.mpr h).trans (interior_mono subset_closure)
-#align is_open.interior_closure_subset Rubin.RegularSupport.IsOpen.interiorClosure_subset
-
-theorem RegularSupport.regular_interior_closure (U : Set α) :
-    is_regular_open (Rubin.RegularSupport.InteriorClosure U) :=
-  by
-  rw [is_regular_def]
-  apply Set.Subset.antisymm
-  exact interior_mono ((closure_mono interior_subset).trans (subset_of_eq closure_closure))
-  exact (subset_of_eq interior_interior.symm).trans (interior_mono subset_closure)
-#align regular_interior_closure Rubin.RegularSupport.regular_interior_closure
-
-def RegularSupport.RegularSupport (α : Type _) [TopologicalSpace α] [MulAction G α] (g : G) :=
-  Rubin.RegularSupport.InteriorClosure (Support α g)
-#align regular_support Rubin.RegularSupport.RegularSupport
-
-theorem RegularSupport.regularSupport_regular {g : G} :
-    is_regular_open (Rubin.RegularSupport.RegularSupport α g) :=
-  Rubin.RegularSupport.regular_interior_closure _
-#align regular_regular_support Rubin.RegularSupport.regularSupport_regular
-
-theorem RegularSupport.support_subset_regularSupport [T2Space α] (g : G) :
-    Support α g ⊆ Rubin.RegularSupport.RegularSupport α g :=
-  Rubin.RegularSupport.IsOpen.interiorClosure_subset (Rubin.support_open g)
-#align support_in_regular_support Rubin.RegularSupport.support_subset_regularSupport
-
-theorem RegularSupport.mem_regularSupport (g : G) (U : Set α) :
-    is_regular_open U → g ∈ RigidStabilizer G U → Rubin.RegularSupport.RegularSupport α g ⊆ U :=
-  fun U_ro g_moves =>
-  (is_regular_def _).mp U_ro ▸
-    Rubin.RegularSupport.interiorClosure_mono (rist_supported_in_set g_moves)
-#align mem_regular_support Rubin.RegularSupport.mem_regularSupport
-
--- FIXME: Weird naming?
-def RegularSupport.AlgebraicCentralizer (f : G) : Set G :=
-  {h | ∃ g, h = g ^ 12 ∧ AlgebraicallyDisjoint f g}
-#align algebraic_centralizer Rubin.RegularSupport.AlgebraicCentralizer
-
-end Rubin.RegularSupport.RegularSupport
+end proposition_2_1
 
 -- ----------------------------------------------------------------
 -- section finite_exponent

Rubin/AlgebraicDisjointness.lean

@@ -88,6 +88,10 @@ fun (h : G) (nc : ¬Commute f h) => {
     exact (mk_thm h nc).choose_spec.choose_spec.right.right.right
 }
 
+-- This is an idea of having a Prop version of AlgebraicallyDisjoint, but it sounds painful to work with
+-- def IsAlgebraicallyDisjoint {G : Type _} [Group G] (f g : G): Prop :=
+--   ∀ (h : G), ¬Commute f h → ∃ (f₁ f₂ : G), ∃ (elem : AlgebraicallyDisjointElem f g h), elem.fst = f₁ ∧ elem.snd = f₂
+
 @[simp]
 theorem orbit_bot (G : Type _) [Group G] [MulAction G α] (p : α) :
     MulAction.orbit (⊥ : Subgroup G) p = {p} :=
@@ -190,6 +194,7 @@ by
       <;> simp only [h, true_or, or_true]
   · rintro ((h|h)|(h|h)) <;> exact ⟨_, h⟩
 
+-- TODO: move to Rubin.lean
 -- TODO: modify the proof to be less "let everything"-y, especially the first half
 -- TODO: use the new class thingy to write a cleaner proof?
 lemma proposition_1_1_1 [h_lm : LocallyMoving G α] [T2Space α] (f g : G) (supp_disjoint : Disjoint (Support α f) (Support α g)) : AlgebraicallyDisjoint f g := by
@@ -208,7 +213,7 @@ lemma proposition_1_1_1 [h_lm : LocallyMoving G α] [T2Space α] (f g : G) (supp
 
   -- Re-use the Hausdoff property of α again, this time yielding W ⊆ V
   let ⟨y, y_moved⟩ := faithful_moves_point' α f₂_ne_one
-  have y_in_V := (rist_supported_in_set f₂_in_rist_V) (mem_support.mpr y_moved)
+  have y_in_V := (rigidStabilizer_support.mp f₂_in_rist_V) (mem_support.mpr y_moved)
   let ⟨W, W_open, y_in_W, W_in_V, disjoint_img_W⟩ := disjoint_nbhd_in V_open y_in_V y_moved
 
   -- Let f₁ be a nontrivial element of RigidStabilizer G W
@@ -220,13 +225,13 @@ lemma proposition_1_1_1 [h_lm : LocallyMoving G α] [T2Space α] (f g : G) (supp
   · apply disjoint_commute (α := α)
     apply Set.disjoint_of_subset_left _ supp_disjoint
     calc
-      Support α f₁ ⊆ W := rist_supported_in_set f₁_in_rist_W
+      Support α f₁ ⊆ W := rigidStabilizer_support.mp f₁_in_rist_W
       W ⊆ V := W_in_V
       V ⊆ Support α f := V_in_support
   · apply disjoint_commute (α := α)
     apply Set.disjoint_of_subset_left _ supp_disjoint
     calc
-      Support α f₂ ⊆ V := rist_supported_in_set f₂_in_rist_V
+      Support α f₂ ⊆ V := rigidStabilizer_support.mp f₂_in_rist_V
       V ⊆ Support α f := V_in_support
 
   -- We claim that [f₁, [f₂, h]] is a nontrivial elelement of Centralizer G g
@@ -237,14 +242,14 @@ lemma proposition_1_1_1 [h_lm : LocallyMoving G α] [T2Space α] (f g : G) (supp
     symm
     apply disjoint_support_comm f₂ h _ disjoint_img_V
     · exact W_in_V z_in_W
-    · exact rist_supported_in_set f₂_in_rist_V
+    · exact rigidStabilizer_support.mp f₂_in_rist_V
 
   constructor
   · -- then `k*f₁⁻¹*k⁻¹` is supported on k W = f₂ W,
     -- so [f₁,k] is supported on W ∪ f₂ W ⊆ V ⊆ support f, so commutes with g.
     apply disjoint_commute (α := α)
     apply Set.disjoint_of_subset_left _ supp_disjoint
-    have supp_f₁_subset_W := (rist_supported_in_set f₁_in_rist_W)
+    have supp_f₁_subset_W := (rigidStabilizer_support.mp f₁_in_rist_W)
 
     show Support α ⁅f₁, ⁅f₂, h⁆⁆ ⊆ Support α f
     calc
@@ -259,14 +264,14 @@ lemma proposition_1_1_1 [h_lm : LocallyMoving G α] [T2Space α] (f g : G) (supp
       _ ⊆ V ∪ V := by
         apply Set.union_subset_union_right
         apply smulImage_subset_in_support f₂ W V W_in_V
-        exact rist_supported_in_set f₂_in_rist_V
+        exact rigidStabilizer_support.mp f₂_in_rist_V
       _ ⊆ V := by rw [Set.union_self]
       _ ⊆ Support α f := V_in_support
 
   · -- finally, [f₁,k] agrees with f₁ on W, so is not the identity.
     have h₄: ∀ z ∈ W, ⁅f₁, k⁆ • z = f₁ • z := by
       apply disjoint_support_comm f₁ k
-      exact rist_supported_in_set f₁_in_rist_W
+      exact rigidStabilizer_support.mp f₁_in_rist_W
       rw [<-smulImage_eq_of_smul_eq h₂]
       exact disjoint_img_W
     let ⟨z, z_in_W, z_moved⟩ := faithful_rigid_stabilizer_moves_point f₁_in_rist_W f₁_ne_one
@@ -522,7 +527,7 @@ by
     symm
 
     apply disjoint_support_comm h f
-    · exact rist_supported_in_set h_in_ristV
+    · exact rigidStabilizer_support.mp h_in_ristV
     · exact V_disjoint_smulImage
     · exact z_in_V
 
@@ -541,11 +546,11 @@ by
       Support α ⁅f₂, h⁆ ⊆ Support α h ∪ (f₂ •'' Support α h) := support_comm α f₂ h
       _ ⊆ V ∪ (f₂ •'' Support α h) := by
         apply Set.union_subset_union_left
-        exact rist_supported_in_set h_in_ristV
+        exact rigidStabilizer_support.mp h_in_ristV
       _ ⊆ V ∪ (f₂ •'' V) := by
         apply Set.union_subset_union_right
         apply smulImage_subset
-        exact rist_supported_in_set h_in_ristV
+        exact rigidStabilizer_support.mp h_in_ristV
   have support_h' : Support α h' ⊆ ⋃(i : Fin 2 × Fin 2), (f₁^(i.1.val) * f₂^(i.2.val)) •'' V := by
     rw [rewrite_Union]
     simp (config := {zeta := false})

Rubin/FaithfulAction.lean

@@ -29,7 +29,7 @@ theorem faithful_rigid_stabilizer_moves_point {g : G} {U : Set α} :
   by
   intro g_rigid g_ne_one
   let ⟨x, xmoved⟩ := Rubin.faithful_moves_point' α g_ne_one
-  exact ⟨x, rist_supported_in_set g_rigid xmoved, xmoved⟩
+  exact ⟨x, rigidStabilizer_support.mp g_rigid xmoved, xmoved⟩
 #align faithful_rist_moves_point Rubin.faithful_rigid_stabilizer_moves_point
 
 theorem ne_one_support_nonempty {g : G} : g ≠ 1 → (Support α g).Nonempty :=

Rubin/OrbitClosure.lean

@@ -1,12 +0,0 @@
-import Mathlib.GroupTheory.GroupAction.Basic
-import Mathlib.GroupTheory.Subgroup.Basic
-import Mathlib.Data.Finset.Basic
-
-import Rubin.Support
-
-namespace Rubin
-
-def OrbitClosure {G α : Type _} [Group G] [MulAction G α] (U : Set α) :=
-  { g : G | Support α g ⊆ U}
-
-end Rubin

Rubin/RegularSupport.lean

@@ -0,0 +1,169 @@
+import Mathlib.Data.Finset.Basic
+import Mathlib.GroupTheory.Commutator
+import Mathlib.GroupTheory.GroupAction.Basic
+import Mathlib.Topology.Basic
+import Mathlib.Topology.Separation
+
+import Rubin.Tactic
+import Rubin.Support
+import Rubin.Topological
+
+namespace Rubin
+
+section InteriorClosure
+
+variable {α : Type _}
+variable [TopologicalSpace α]
+
+-- Defines a kind of "regularization" transformation made to sets,
+-- by calling `closure` followed by `interior` on it.
+--
+-- A set is then said to be regular if the InteriorClosure does not modify it.
+def InteriorClosure (U : Set α) : Set α :=
+  interior (closure U)
+#align interior_closure Rubin.InteriorClosure
+
+@[simp]
+theorem InteriorClosure.def (U : Set α) : InteriorClosure U = interior (closure U) :=
+  by simp [InteriorClosure]
+
+@[simp]
+theorem InteriorClosure.fdef : InteriorClosure = (interior ∘ (closure (α := α))) := by ext; simp
+
+def Regular (U : Set α) : Prop :=
+  InteriorClosure U = U
+
+@[simp]
+theorem Regular.def (U : Set α) : Regular U ↔ interior (closure U) = U :=
+  by simp [Regular]
+#align set.is_regular_def Rubin.Regular.def
+
+theorem interiorClosure_open (U : Set α) : IsOpen (InteriorClosure U) := by simp
+#align is_open_interior_closure Rubin.interiorClosure_open
+
+theorem interiorClosure_subset {U : Set α} :
+  IsOpen U → U ⊆ InteriorClosure U :=
+by
+  intro h
+  apply subset_trans
+  exact subset_interior_iff_isOpen.mpr h
+  apply interior_mono
+  exact subset_closure
+#align is_open.interior_closure_subset Rubin.interiorClosure_subset
+
+theorem interiorClosure_regular (U : Set α) : Regular (InteriorClosure U) :=
+by
+  apply Set.eq_of_subset_of_subset <;> unfold InteriorClosure
+  {
+    apply interior_mono
+    nth_rw 2 [<-closure_closure (s := U)]
+    apply closure_mono
+    exact interior_subset
+  }
+  {
+    nth_rw 1 [<-interior_interior]
+    apply interior_mono
+    exact subset_closure
+  }
+#align regular_interior_closure Rubin.interiorClosure_regular
+
+theorem interiorClosure_mono (U V : Set α) :
+  U ⊆ V → InteriorClosure U ⊆ InteriorClosure V
+:= interior_mono ∘ closure_mono
+#align interior_closure_mono Rubin.interiorClosure_mono
+
+theorem monotone_interior_closure : Monotone (InteriorClosure (α := α))
+:= fun a b =>
+  interiorClosure_mono a b
+
+theorem regular_open (U : Set α) : Regular U → IsOpen U :=
+by
+  intro h_reg
+  rw [<-h_reg]
+  simp
+
+theorem regular_interior (U : Set α) : Regular U → interior U = U :=
+by
+  intro h_reg
+  rw [<-h_reg]
+  simp
+
+end InteriorClosure
+
+section RegularSupport_def
+
+variable {G : Type _}
+variable (α : Type _)
+variable [Group G]
+variable [MulAction G α]
+variable [TopologicalSpace α]
+
+-- The "regular support" is the `Support` of `g : G` in `α` (aka the elements in `α` which are moved by `g`),
+-- transformed with the interior closure.
+def RegularSupport (g: G) : Set α :=
+  InteriorClosure (Support α g)
+#align regular_support Rubin.RegularSupport
+
+@[simp]
+theorem RegularSupport.def {G : Type _} (α : Type _)
+  [Group G] [MulAction G α] [TopologicalSpace α]
+  (g: G) : RegularSupport α g = interior (closure (Support α g)) :=
+by
+  simp [RegularSupport]
+
+theorem regularSupport_open [MulAction G α] (g : G) : IsOpen (RegularSupport α g) := by
+  unfold RegularSupport
+  simp
+
+theorem regularSupport_regular [MulAction G α] (g : G) : Regular (RegularSupport α g) := by
+  apply interiorClosure_regular
+#align regular_regular_support Rubin.regularSupport_regular
+
+end RegularSupport_def
+
+section RegularSupport_continuous
+
+variable {G α : Type _}
+variable [Group G]
+variable [TopologicalSpace α]
+variable [ContinuousMulAction G α]
+
+theorem support_subset_regularSupport [T2Space α] {g : G} :
+  Support α g ⊆ RegularSupport α g :=
+by
+  apply interiorClosure_subset
+  apply support_open (α := α) (g := g)
+#align support_in_regular_support Rubin.support_subset_regularSupport
+
+theorem regularSupport_subset {g : G} {U : Set α} :
+  Regular U → Support α g ⊆ U → RegularSupport α g ⊆ U :=
+by
+  intro U_reg h
+  rw [<-U_reg]
+  apply interiorClosure_mono
+  exact h
+
+theorem regularSupport_subset_of_rigidStabilizer {g : G} {U : Set α} (U_reg : Regular U) :
+  g ∈ RigidStabilizer G U → RegularSupport α g ⊆ U :=
+by
+  intro g_in_rist
+  apply regularSupport_subset
+  · assumption
+  · apply rigidStabilizer_support.mp
+    assumption
+
+theorem regularSupport_subset_iff_rigidStabilizer [T2Space α] {g : G} {U : Set α} (U_reg : Regular U) :
+  g ∈ RigidStabilizer G U ↔ RegularSupport α g ⊆ U :=
+by
+  constructor
+  · apply regularSupport_subset_of_rigidStabilizer U_reg
+  · intro rs_subset
+    rw [rigidStabilizer_support]
+    apply subset_trans
+    apply support_subset_regularSupport
+    exact rs_subset
+#align mem_regular_support Rubin.regularSupport_subset_of_rigidStabilizer
+
+end RegularSupport_continuous
+
+end Rubin

Rubin/RigidStabilizer.lean

@@ -10,6 +10,7 @@ def rigidStabilizer' (G : Type _) [Group G] [MulAction G α] (U : Set α) : Set
   {g : G | ∀ x : α, g • x = x ∨ x ∈ U}
 #align rigid_stabilizer' Rubin.rigidStabilizer'
 
+-- A subgroup of G for which `Support α g ⊆ U`, or in other words, all elements of `G` that don't move points outside of `U`.
 def RigidStabilizer (G : Type _) [Group G] [MulAction G α] (U : Set α) : Subgroup G
     where
   carrier := {g : G | ∀ (x) (_ : x ∉ U), g • x = x}
@@ -22,17 +23,40 @@ variable {G α: Type _}
 variable [Group G]
 variable [MulAction G α]
 
-theorem rist_supported_in_set {g : G} {U : Set α} :
-    g ∈ RigidStabilizer G U → Support α g ⊆ U := fun h x x_in_support =>
-  by_contradiction (x_in_support ∘ h x)
-#align rist_supported_in_set Rubin.rist_supported_in_set
-
-theorem rist_ss_rist {U V : Set α} (V_ss_U : V ⊆ U) :
-    (RigidStabilizer G V : Set G) ⊆ (RigidStabilizer G U : Set G) :=
+theorem rigidStabilizer_support {g : G} {U : Set α} :
+  g ∈ RigidStabilizer G U ↔ Support α g ⊆ U :=
+⟨
+  fun h x x_in_support =>
+    by_contradiction (x_in_support ∘ h x),
   by
+    intro support_sub
+    rw [<-Subgroup.mem_carrier]
+    unfold RigidStabilizer; simp
+    intro x x_notin_U
+    by_contra h
+    exact x_notin_U (support_sub h)
+⟩
+#align rist_supported_in_set Rubin.rigidStabilizer_support
+
+theorem rigidStabilizer_mono {U V : Set α} (V_ss_U : V ⊆ U) :
+  (RigidStabilizer G V : Set G) ⊆ (RigidStabilizer G U : Set G) :=
+by
   intro g g_in_ristV x x_notin_U
   have x_notin_V : x ∉ V := by intro x_in_V; exact x_notin_U (V_ss_U x_in_V)
   exact g_in_ristV x x_notin_V
-#align rist_ss_rist Rubin.rist_ss_rist
+#align rist_ss_rist Rubin.rigidStabilizer_mono
+
+theorem monotone_rigidStabilizer : Monotone (RigidStabilizer (α := α) G) := fun _ _ => rigidStabilizer_mono
+
+theorem rigidStabilizer_to_subgroup_closure {g : G} {U : Set α} :
+  g ∈ RigidStabilizer G U → g ∈ Subgroup.closure { g : G | Support α g ⊆ U } :=
+by
+  rw [rigidStabilizer_support]
+  intro h
+  rw [Subgroup.mem_closure]
+  intro V orbit_subset_V
+  apply orbit_subset_V
+  simp
+  exact h
 
 end Rubin

Rubin/Topological.lean

@@ -12,6 +12,7 @@ namespace Rubin
 
 section Continuity
 
+-- TODO: don't have this extend MulAction
 class ContinuousMulAction (G α : Type _) [Group G] [TopologicalSpace α] extends
     MulAction G α where
   continuous : ∀ g : G, Continuous (fun x: α => g • x)
@@ -92,18 +93,13 @@ class LocallyDense (G α : Type _) [Group G] [TopologicalSpace α] extends MulAc
     p ∈ interior (closure (MulAction.orbit (RigidStabilizer G U) p))
 #align is_locally_dense Rubin.LocallyDense
 
-namespace LocallyDense
-
-lemma nonEmpty {G α : Type _} [Group G] [TopologicalSpace α] [LocallyDense G α]:
+lemma LocallyDense.nonEmpty {G α : Type _} [Group G] [TopologicalSpace α] [LocallyDense G α]:
   ∀ {U : Set α},
   Set.Nonempty U →
   ∃ p ∈ U, p ∈ interior (closure (MulAction.orbit (RigidStabilizer G U) p)) := by
   intros U H_ne
   exact ⟨H_ne.some, H_ne.some_mem, LocallyDense.isLocallyDense U H_ne.some H_ne.some_mem⟩
 
-end LocallyDense
-
-
 end Other
 
 end Rubin