feat: define complete sublattices (#9763)

Mathlib.lean

@@ -2886,6 +2886,7 @@ import Mathlib.Order.CompleteBooleanAlgebra
 import Mathlib.Order.CompleteLattice
 import Mathlib.Order.CompleteLatticeIntervals
 import Mathlib.Order.CompletePartialOrder
+import Mathlib.Order.CompleteSublattice
 import Mathlib.Order.Concept
 import Mathlib.Order.ConditionallyCompleteLattice.Basic
 import Mathlib.Order.ConditionallyCompleteLattice.Finset

Mathlib/Data/Set/Image.lean

@@ -596,6 +596,12 @@ theorem image_eq_image {f : α → β} (hf : Injective f) : f '' s = f '' t ↔
       rw [← preimage_image_eq s hf, ← preimage_image_eq t hf, eq]
 #align set.image_eq_image Set.image_eq_image
 
+theorem subset_image_iff {t : Set β} :
+    t ⊆ f '' s ↔ ∃ u, u ⊆ s ∧ f '' u = t := by
+  refine ⟨fun h ↦ ⟨f ⁻¹' t ∩ s, inter_subset_right _ _, ?_⟩,
+    fun ⟨u, hu, hu'⟩ ↦ hu'.symm ▸ image_mono hu⟩
+  rwa [image_preimage_inter, inter_eq_left]
+
 theorem image_subset_image_iff {f : α → β} (hf : Injective f) : f '' s ⊆ f '' t ↔ s ⊆ t := by
   refine' Iff.symm <| (Iff.intro (image_subset f)) fun h => _
   rw [← preimage_image_eq s hf, ← preimage_image_eq t hf]

Mathlib/Order/CompleteSublattice.lean

@@ -0,0 +1,125 @@
+/-
+Copyright (c) 2023 Oliver Nash. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Oliver Nash
+-/
+import Mathlib.Order.Sublattice
+import Mathlib.Order.Hom.CompleteLattice
+
+/-!
+# Complete Sublattices
+
+This file defines complete sublattices. These are subsets of complete lattices which are closed
+under arbitrary suprema and infima. As a standard example one could take the complete sublattice of
+invariant submodules of some module with respect to a linear map.
+
+## Main definitions:
+  * `CompleteSublattice`: the definition of a complete sublattice
+  * `CompleteSublattice.mk'`: an alternate constructor for a complete sublattice, demanding fewer
+    hypotheses
+  * `CompleteSublattice.instCompleteLattice`: a complete sublattice is a complete lattice
+  * `CompleteSublattice.map`: complete sublattices push forward under complete lattice morphisms.
+  * `CompleteSublattice.comap`: complete sublattices pull back under complete lattice morphisms.
+
+-/
+
+open Function Set
+
+variable (α β : Type*) [CompleteLattice α] [CompleteLattice β] (f : CompleteLatticeHom α β)
+
+/-- A complete sublattice is a subset of a complete lattice that is closed under arbitrary suprema
+and infima. -/
+structure CompleteSublattice extends Sublattice α where
+  sSupClosed' : ∀ ⦃s : Set α⦄, s ⊆ carrier → sSup s ∈ carrier
+  sInfClosed' : ∀ ⦃s : Set α⦄, s ⊆ carrier → sInf s ∈ carrier
+
+variable {α β}
+
+namespace CompleteSublattice
+
+/-- To check that a subset is a complete sublattice, one does not need to check that it is closed
+under binary `Sup` since this follows from the stronger `sSup` condition. Likewise for infima. -/
+@[simps] def mk' (carrier : Set α)
+    (sSupClosed' : ∀ ⦃s : Set α⦄, s ⊆ carrier → sSup s ∈ carrier)
+    (sInfClosed' : ∀ ⦃s : Set α⦄, s ⊆ carrier → sInf s ∈ carrier) :
+  CompleteSublattice α where
+    carrier := carrier
+    sSupClosed' := sSupClosed'
+    sInfClosed' := sInfClosed'
+    supClosed' := fun x hx y hy ↦ by
+      suffices x ⊔ y = sSup {x, y} by exact this ▸ sSupClosed' (fun z hz ↦ by aesop)
+      simp [sSup_singleton]
+    infClosed' := fun x hx y hy ↦ by
+      suffices x ⊓ y = sInf {x, y} by exact this ▸ sInfClosed' (fun z hz ↦ by aesop)
+      simp [sInf_singleton]
+
+variable {L : CompleteSublattice α}
+
+instance instSetLike : SetLike (CompleteSublattice α) α where
+  coe L := L.carrier
+  coe_injective' L M h := by cases L; cases M; congr; exact SetLike.coe_injective' h
+
+instance instBot : Bot L where
+  bot := ⟨⊥, by simpa using L.sSupClosed' <| empty_subset _⟩
+
+instance instTop : Top L where
+  top := ⟨⊤, by simpa using L.sInfClosed' <| empty_subset _⟩
+
+instance instSupSet : SupSet L where
+  sSup s := ⟨sSup s, L.sSupClosed' image_val_subset⟩
+
+instance instInfSet : InfSet L where
+  sInf s := ⟨sInf s, L.sInfClosed' image_val_subset⟩
+
+theorem sSupClosed {s : Set α} (h : s ⊆ L) : sSup s ∈ L := L.sSupClosed' h
+
+theorem sInfClosed {s : Set α} (h : s ⊆ L) : sInf s ∈ L := L.sInfClosed' h
+
+@[simp] theorem coe_bot : (↑(⊥ : L) : α) = ⊥ := rfl
+
+@[simp] theorem coe_top : (↑(⊤ : L) : α) = ⊤ := rfl
+
+@[simp] theorem coe_sSup (S : Set L) : (↑(sSup S) : α) = sSup {(s : α) | s ∈ S} := rfl
+
+theorem coe_sSup' (S : Set L) : (↑(sSup S) : α) = ⨆ N ∈ S, (N : α) := by
+  rw [coe_sSup, ← Set.image, sSup_image]
+
+@[simp] theorem coe_sInf (S : Set L) : (↑(sInf S) : α) = sInf {(s : α) | s ∈ S} := rfl
+
+theorem coe_sInf' (S : Set L) : (↑(sInf S) : α) = ⨅ N ∈ S, (N : α) := by
+  rw [coe_sInf, ← Set.image, sInf_image]
+
+instance instCompleteLattice : CompleteLattice L :=
+  Subtype.coe_injective.completeLattice _
+    Sublattice.coe_sup Sublattice.coe_inf coe_sSup' coe_sInf' coe_top coe_bot
+
+/-- The push forward of a complete sublattice under a complete lattice hom is a complete
+sublattice. -/
+@[simps] def map (L : CompleteSublattice α) : CompleteSublattice β where
+  carrier := f '' L
+  supClosed' := L.supClosed.image f
+  infClosed' := L.infClosed.image f
+  sSupClosed' := fun s hs ↦ by
+    obtain ⟨t, ht, rfl⟩ := subset_image_iff.mp hs
+    rw [← map_sSup]
+    exact mem_image_of_mem f (sSupClosed ht)
+  sInfClosed' := fun s hs ↦ by
+    obtain ⟨t, ht, rfl⟩ := subset_image_iff.mp hs
+    rw [← map_sInf]
+    exact mem_image_of_mem f (sInfClosed ht)
+
+@[simp] theorem mem_map {b : β} : b ∈ L.map f ↔ ∃ a ∈ L, f a = b := Iff.rfl
+
+/-- The pull back of a complete sublattice under a complete lattice hom is a complete sublattice. -/
+@[simps] def comap (L : CompleteSublattice β) : CompleteSublattice α where
+  carrier := f ⁻¹' L
+  supClosed' := L.supClosed.preimage f
+  infClosed' := L.infClosed.preimage f
+  sSupClosed' s hs := by
+    simpa only [mem_preimage, map_sSup, SetLike.mem_coe] using sSupClosed <| mapsTo'.mp hs
+  sInfClosed' s hs := by
+    simpa only [mem_preimage, map_sInf, SetLike.mem_coe] using sInfClosed <| mapsTo'.mp hs
+
+@[simp] theorem mem_comap {L : CompleteSublattice β} {a : α} : a ∈ L.comap f ↔ f a ∈ L := Iff.rfl
+
+end CompleteSublattice