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import Mathlib.Data.Finset.Basic
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import Mathlib.GroupTheory.Subgroup.Basic
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import Mathlib.GroupTheory.GroupAction.Basic
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import Rubin.MulActionExt
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namespace Rubin
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/--
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The image of a group action (here generalized to any pair `(G, α)` implementing `SMul`)
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is the image of the elements of `U` under the `g • u` operation.
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An alternative definition (which is available through the [`mem_smulImage`] theorem and the [`smulImage_set`] equality) would be:
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`SmulImage g U = {x | g⁻¹ • x ∈ U}`.
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The notation used for this operator is `g •'' U`.
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-/
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def SmulImage {G α : Type _} [SMul G α] (g : G) (U : Set α) :=
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(g • ·) '' U
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#align subset_img Rubin.SmulImage
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infixl:60 " •'' " => Rubin.SmulImage
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/--
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The pre-image of a group action (here generalized to any pair `(G, α)` implementing `SMul`)
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is the set of values `x: α` such that `g • x ∈ U`.
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Unlike [`SmulImage`], no notation is defined for this operator.
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--/
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def SmulPreImage {G α : Type _} [SMul G α] (g : G) (U : Set α) :=
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{x | g • x ∈ U}
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#align subset_preimg' Rubin.SmulPreImage
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variable {G α : Type _}
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variable [Group G]
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variable [MulAction G α]
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theorem smulImage_def {g : G} {U : Set α} : g •'' U = (· • ·) g '' U :=
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rfl
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#align subset_img_def Rubin.smulImage_def
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theorem mem_smulImage {x : α} {g : G} {U : Set α} : x ∈ g •'' U ↔ g⁻¹ • x ∈ U :=
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by
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rw [Rubin.smulImage_def, Set.mem_image (g • ·) U x]
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constructor
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· rintro ⟨y, yU, gyx⟩
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let ygx : y = g⁻¹ • x := inv_smul_smul g y ▸ Rubin.smul_congr g⁻¹ gyx
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exact ygx ▸ yU
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· intro h
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exact ⟨g⁻¹ • x, ⟨Set.mem_preimage.mp h, smul_inv_smul g x⟩⟩
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#align mem_smul'' Rubin.mem_smulImage
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-- Provides a way to express a [`SmulImage`] as a `Set`;
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-- this is simply [`mem_smulImage`] paired with set extensionality.
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theorem smulImage_set {g: G} {U: Set α} : g •'' U = {x | g⁻¹ • x ∈ U} := Set.ext (fun _x => mem_smulImage)
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theorem mem_inv_smulImage {x : α} {g : G} {U : Set α} : x ∈ g⁻¹ •'' U ↔ g • x ∈ U :=
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by
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let msi := @Rubin.mem_smulImage _ _ _ _ x g⁻¹ U
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rw [inv_inv] at msi
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exact msi
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#align mem_inv_smul'' Rubin.mem_inv_smulImage
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-- TODO: rename to smulImage_mul
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@[simp]
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theorem mul_smulImage (g h : G) (U : Set α) : g •'' (h •'' U) = (g * h) •'' U :=
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by
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ext
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rw [Rubin.mem_smulImage, Rubin.mem_smulImage, Rubin.mem_smulImage, ←
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mul_smul, mul_inv_rev]
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#align mul_smul'' Rubin.mul_smulImage
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@[simp]
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theorem one_smulImage (U : Set α) : (1 : G) •'' U = U :=
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by
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ext
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rw [Rubin.mem_smulImage, inv_one, one_smul]
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#align one_smul'' Rubin.one_smulImage
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theorem disjoint_smulImage (g : G) {U V : Set α} :
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Disjoint U V → Disjoint (g •'' U) (g •'' V) :=
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by
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intro disjoint_U_V
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rw [Set.disjoint_left]
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rw [Set.disjoint_left] at disjoint_U_V
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intro x x_in_gU
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by_contra h
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exact (disjoint_U_V (mem_smulImage.mp x_in_gU)) (mem_smulImage.mp h)
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#align disjoint_smul'' Rubin.disjoint_smulImage
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namespace SmulImage
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theorem congr (g : G) {U V : Set α} : U = V → g •'' U = g •'' V :=
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congr_arg fun W : Set α => g •'' W
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#align smul''_congr Rubin.SmulImage.congr
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end SmulImage
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theorem smulImage_subset (g : G) {U V : Set α} : U ⊆ V → g •'' U ⊆ g •'' V :=
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by
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intro h1 x
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rw [Rubin.mem_smulImage, Rubin.mem_smulImage]
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exact fun h2 => h1 h2
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#align smul''_subset Rubin.smulImage_subset
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theorem smulImage_union (g : G) {U V : Set α} : g •'' U ∪ V = (g •'' U) ∪ (g •'' V) :=
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by
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ext
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rw [Rubin.mem_smulImage, Set.mem_union, Set.mem_union, Rubin.mem_smulImage,
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Rubin.mem_smulImage]
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#align smul''_union Rubin.smulImage_union
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theorem smulImage_inter (g : G) {U V : Set α} : g •'' U ∩ V = (g •'' U) ∩ (g •'' V) :=
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by
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ext
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rw [Set.mem_inter_iff, Rubin.mem_smulImage, Rubin.mem_smulImage,
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Rubin.mem_smulImage, Set.mem_inter_iff]
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#align smul''_inter Rubin.smulImage_inter
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theorem smulImage_eq_inv_preimage {g : G} {U : Set α} : g •'' U = (g⁻¹ • ·) ⁻¹' U :=
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by
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ext
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constructor
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· intro h; rw [Set.mem_preimage]; exact mem_smulImage.mp h
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· intro h; rw [Rubin.mem_smulImage]; exact Set.mem_preimage.mp h
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#align smul''_eq_inv_preimage Rubin.smulImage_eq_inv_preimage
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theorem smulImage_eq_of_smul_eq {g h : G} {U : Set α} :
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(∀ x ∈ U, g • x = h • x) → g •'' U = h •'' U :=
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by
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intro hU
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ext x
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rw [Rubin.mem_smulImage, Rubin.mem_smulImage]
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constructor
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· intro k; let a := congr_arg (h⁻¹ • ·) (hU (g⁻¹ • x) k);
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simp only [smul_inv_smul, inv_smul_smul] at a ; exact Set.mem_of_eq_of_mem a k
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· intro k; let a := congr_arg (g⁻¹ • ·) (hU (h⁻¹ • x) k);
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simp only [smul_inv_smul, inv_smul_smul] at a ; exact Set.mem_of_eq_of_mem a.symm k
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#align smul''_eq_of_smul_eq Rubin.smulImage_eq_of_smul_eq
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theorem smulImage_subset_inv {G α : Type _} [Group G] [MulAction G α]
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(f : G) (U V : Set α) :
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f •'' U ⊆ V ↔ U ⊆ f⁻¹ •'' V :=
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by
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constructor
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· intro h
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apply smulImage_subset f⁻¹ at h
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rw [mul_smulImage] at h
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rw [mul_left_inv, one_smulImage] at h
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exact h
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· intro h
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apply smulImage_subset f at h
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rw [mul_smulImage] at h
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rw [mul_right_inv, one_smulImage] at h
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exact h
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theorem smulImage_subset_inv' {G α : Type _} [Group G] [MulAction G α]
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(f : G) (U V : Set α) :
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f⁻¹ •'' U ⊆ V ↔ U ⊆ f •'' V :=
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by
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nth_rewrite 2 [<-inv_inv f]
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exact smulImage_subset_inv f⁻¹ U V
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theorem smulImage_disjoint_mul {G α : Type _} [Group G] [MulAction G α]
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(f g : G) (U V : Set α) :
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Disjoint (f •'' U) (g •'' V) ↔ Disjoint U ((f⁻¹ * g) •'' V) := by
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constructor
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· intro h
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apply disjoint_smulImage f⁻¹ at h
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repeat rw [mul_smulImage] at h
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rw [mul_left_inv, one_smulImage] at h
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exact h
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· intro h
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apply disjoint_smulImage f at h
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rw [mul_smulImage] at h
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rw [<-mul_assoc] at h
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rw [mul_right_inv, one_mul] at h
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exact h
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theorem smulImage_disjoint_inv_pow {G α : Type _} [Group G] [MulAction G α]
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(g : G) (i j : ℤ) (U V : Set α) :
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Disjoint (g^i •'' U) (g^j •'' V) ↔ Disjoint (g^(-j) •'' U) (g^(-i) •'' V) :=
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by
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rw [smulImage_disjoint_mul]
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rw [<-zpow_neg, <-zpow_add, add_comm, zpow_add, zpow_neg]
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rw [<-inv_inv (g^j)]
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rw [<-smulImage_disjoint_mul]
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simp
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theorem smulImage_disjoint_subset {G α : Type _} [Group G] [MulAction G α]
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{f g : G} {U V : Set α} (h_sub: U ⊆ V):
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Disjoint (f •'' V) (g •'' V) → Disjoint (f •'' U) (g •'' U) :=
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by
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apply Set.disjoint_of_subset (smulImage_subset _ h_sub) (smulImage_subset _ h_sub)
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end Rubin
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