feat(order): closure operators (#5524)

Adds closure operators on a partial order, as in [wikipedia](https://en.wikipedia.org/wiki/Closure_operator#Closure_operators_on_partially_ordered_sets). I made them bundled for no particular reason, I don't mind unbundling.

src/order/closure.lean

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+/-
+Copyright (c) 2020 Bhavik Mehta. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Author: Bhavik Mehta
+-/
+import order.basic
+import order.preorder_hom
+import order.galois_connection
+import tactic.monotonicity
+
+/-!
+# Closure operators on a partial order
+
+We define (bundled) closure operators on a partial order as an monotone (increasing), extensive
+(inflationary) and idempotent function.
+We define closed elements for the operator as elements which are fixed by it.
+
+Note that there is close connection to Galois connections and Galois insertions: every closure
+operator induces a Galois insertion (from the set of closed elements to the underlying type), and
+every Galois connection induces a closure operator (namely the composition). In particular,
+a Galois insertion can be seen as a general case of a closure operator, where the inclusion is given
+by coercion, see `closure_operator.gi`.
+
+## References
+
+* https://en.wikipedia.org/wiki/Closure_operator#Closure_operators_on_partially_ordered_sets
+
+-/
+universe u
+
+variables (α : Type u) [partial_order α]
+
+/--
+A closure operator on the partial order `α` is a monotone function which is extensive (every `x`
+is less than its closure) and idempotent.
+-/
+structure closure_operator extends α →ₘ α :=
+(le_closure' : ∀ x, x ≤ to_fun x)
+(idempotent' : ∀ x, to_fun (to_fun x) = to_fun x)
+
+instance : has_coe_to_fun (closure_operator α) :=
+{ F := _, coe := λ c, c.to_fun }
+
+initialize_simps_projections closure_operator (to_fun → apply)
+
+namespace closure_operator
+
+/-- The identity function as a closure operator. -/
+@[simps]
+def id : closure_operator α :=
+{ to_fun := λ x, x,
+  monotone' := λ _ _ h, h,
+  le_closure' := λ _, le_refl _,
+  idempotent' := λ _, rfl }
+
+instance : inhabited (closure_operator α) := ⟨id α⟩
+
+variables {α} (c : closure_operator α)
+
+@[ext] lemma ext :
+  ∀ (c₁ c₂ : closure_operator α), (c₁ : α → α) = (c₂ : α → α) → c₁ = c₂
+| ⟨⟨c₁, _⟩, _, _⟩ ⟨⟨c₂, _⟩, _, _⟩ h := by { congr, exact h }
+
+@[mono] lemma monotone : monotone c := c.monotone'
+/--
+Every element is less than its closure. This property is sometimes referred to as extensivity or
+inflationary.
+-/
+lemma le_closure (x : α) : x ≤ c x := c.le_closure' x
+@[simp] lemma idempotent (x : α) : c (c x) = c x := c.idempotent' x
+
+lemma le_closure_iff (x y : α) : x ≤ c y ↔ c x ≤ c y :=
+⟨λ h, c.idempotent y ▸ c.monotone h, λ h, le_trans (c.le_closure x) h⟩
+
+lemma closure_top {α : Type u} [order_top α] (c : closure_operator α) : c ⊤ = ⊤ :=
+le_antisymm le_top (c.le_closure _)
+
+lemma closure_inter_le {α : Type u} [semilattice_inf α] (c : closure_operator α) (x y : α) :
+  c (x ⊓ y) ≤ c x ⊓ c y :=
+le_inf (c.monotone inf_le_left) (c.monotone inf_le_right)
+
+lemma closure_union_closure_le {α : Type u} [semilattice_sup α] (c : closure_operator α) (x y : α) :
+  c x ⊔ c y ≤ c (x ⊔ y) :=
+sup_le (c.monotone le_sup_left) (c.monotone le_sup_right)
+
+/-- An element `x` is closed for the closure operator `c` if it is a fixed point for it. -/
+def closed : set α := λ x, c x = x
+
+lemma mem_closed_iff (x : α) : x ∈ c.closed ↔ c x = x := iff.rfl
+lemma mem_closed_iff_closure_le (x : α) : x ∈ c.closed ↔ c x ≤ x :=
+⟨le_of_eq, λ h, le_antisymm h (c.le_closure x)⟩
+lemma closure_eq_self_of_mem_closed {x : α} (h : x ∈ c.closed) : c x = x := h
+
+@[simp] lemma closure_is_closed (x : α) : c x ∈ c.closed := c.idempotent x
+
+/-- The set of closed elements for `c` is exactly its range. -/
+lemma closed_eq_range_close : c.closed = set.range c :=
+set.ext $ λ x, ⟨λ h, ⟨x, h⟩, by { rintro ⟨y, rfl⟩, apply c.idempotent }⟩
+
+/-- Send an `x` to an element of the set of closed elements (by taking the closure). -/
+def to_closed (x : α) : c.closed := ⟨c x, c.closure_is_closed x⟩
+
+lemma top_mem_closed {α : Type u} [order_top α] (c : closure_operator α) : ⊤ ∈ c.closed :=
+c.closure_top
+
+lemma closure_le_closed_iff_le {x y : α} (hy : c.closed y) : x ≤ y ↔ c x ≤ y :=
+by rw [← c.closure_eq_self_of_mem_closed hy, le_closure_iff]
+
+/-- The set of closed elements has a galois insertion to the underlying type. -/
+def gi : galois_insertion c.to_closed coe :=
+{ choice := λ x hx, ⟨x, le_antisymm hx (c.le_closure x)⟩,
+  gc := λ x y, (c.closure_le_closed_iff_le y.2).symm,
+  le_l_u := λ x, c.le_closure _,
+  choice_eq := λ x hx, le_antisymm (c.le_closure x) hx }
+
+end closure_operator